One of the most common questions in this part of VideoForums is "What (cheap) microphone should I buy for video recording?" The answer, given time and time again is "None." The trouble is, some newbies don't want to hear this and keep asking the question in the hope that, miraculously, they will get a different answer.
So, I'm going to try and explain why there's no such thing as a "decent" cheapo microphone.
It doesn't matter which camcorder you buy, the in-built microphone will not be the best solution to recording sound. It's better than nothing, but only just. Even on a £4k camcorder, the manufacturer will only have spent £25 on the microphones. They figure that most serious videographers will buy a separate microphone anyway. So, theoretically, a £50 - £100 mic should sound better than the camera's mic. Unfortunately it doesn't work like this.
Cheap microphones sound like cheap microphones. They are often identical to the capsules used in the camcorder mic, just in a different housing. When you consider that the housing, packaging, transport and import duties will be about 70% of the cost of a sub-£100 mic, you can see how little is left over for the actual capsule and electronics! There is a myth that cheap chinese microphones are identical to more expensive makes, since some of the expensive brands' components are made in China. This isn't the case. What sometimes happens is that components rejected by known brands find their way into the cheap mics. Not an ideal situation by any stretch of the imagination.
In the same way cheapo mics from firms like Vivanco, Hama etc will not improve your sound dramatically. When you work out labour costs (small though they are in some countries) A £50 mic will still only have components worth about a fiver!
One fact should be remembered at all times: To get the best sound, you need to get in close. Distance kills sound. It reduces the clarity and the high tones. A cheap microphone up close will, nearly always, sound better than a good microphone a long way away. Since it's not always practical to have the microphone six inches in front of the speaker's mouth, different types of microphones have been developed. The most common, for video use, is the "shotgun".
The following is meant to let beginners have an idea of how microphones work. Some explanations have been simplified or reduced in order to avoid ten pages of technical descriptions.
There are more misconceptions about shotgun or short-gun microphones than anything else. They are directional, sort of, but are not the aural equivelant of a telephoto lens. They work by rejecting sound from the sides using an interference tube. In essence this tube delays the sounds coming from the sides, or "off-axis" in such a way that they cancel each other out. Where it has difficulties is with middle and lower tones and with echoes. So, a shotgun microphone ten feet from the speaker will still sound "distant" or "roomy". For recording the spoken word at the best quality, even a shotgun should be no more than a yard from the speaker.
It's also worth noting that off axis sounds lose some of their spectrum, resulting in speech sounding distorted if you aim the microphone incorrectly.
The top Shotgun microphones are the Schoeps CMT5U and the Sennheiser MKH60, both costing over £1500. Next, at just under a grand, comes the Sennheiser workhorse the "416", a solid microphone which has been around since the seventies and is still going strong. These microphones will last for ever. One of our most prolific members is using a thirty year old MKH416 and it still sounds superb.
Moving down the scale are the sort of "shotguns" which most enthusiasts or semi-pro videographers should be looking at. The Rode NTG3, the Sennheiser K6 range and the Audio Technica 8035 and 4071 mics all around the £400 - £500 range.
In the budget shotgun section the only serious contender, in my opinion, is the AT897 at around £150. Anything less will sound poor. It doesn't matter how you phrase the question or how many times you ask, the answer remains the same... Cheap microphones sound cheap.
The biggest restriction on your choice of microphone will be the powering required. Most profesional microphones need "48v phantom powering" which professional camcorders deliver through the xlr cable. If your camcorder doesn't deliver 48v phantom power then it doesn't mean that you can't use pro microphones, there are alternatives.
Some mics, such as the Audio Technica AT897 and the Sennheiser K6 system have the ability to power themselves with an AA 1.5 volt battery which fits into the microphone. It results in a slightly weaker signal than if phantom power were available but it means that you can (with an appropriate adapter) use these mics with any camcorder, even those with a 3.5mm socket.
Alternatively you can buy a Location Mixer, a sort of electronic handbag which enables you to adjust the volume and mix various mics together. It's not the sort of thing you can operate at the same time as the camera, so it requires a sound assistant.
Finally you can get a 48v supply. This delivers phantom power to the mic.
If you are starting out and/or don't anticipate going pro in the near future I would recommend the Audio Technica At897 as a good value-for-money shotgun microphone at about £150.
Audio-Technica - Microphones, headphones, wireless microphone systems, noise-cancelling headphones & more : AT897 Line + Gradient Condenser Microphone
If you anticipate moving into semi-pro or drama videomaking then you should be looking at something like the Rode NTG3 or the Sennheiser k6 series. In the past Rode, although known for good value studio microphones, hasn't had a good reputation for video mics. Its chinese-made NTG1 and NTG2 were not at all good however the NTG3 seems to have broken the mould and is getting good reviews for a £500 shotgun microphone.
The Sennheiser range consists of a base unit (the K6) and a selection of capsules which are fitted onto the K6 depending on what type of characteristics you want. This has the advantage that you don't spend as much on a range of microphones as you would buying a non-modular set of mics.The most common capsule is the ME66 shotgun. Sennheiser mics are known for their clarity and nice tonal qualities.
If you're just going to have your microphone attached to your camcorder and only want it to replace the in-built mics for general shooting, then a shotgun is probably a bit too directional for this purpose. You need a "cardiod" or "hypercardiod" microphone. These accept sound from the front and (to some extent) more from the sides than a shotgun. With a narrow pattern (shotgun) mic, anyone off the axis will sound strange as some of the vocal frequencies are filtered out and others remain. It means that the camera has to be pointed directly at the sound-source. Not always possible, or what you want.
The two decent (but not cheap) budget hypercardiods which I recommend are both modular systems. This means that you can change the capsule to another one, depending on your needs. One system is the K6 from Sennheiser. The other, which I prefer, is the "Blueline" series from AKG which is very good indeed and a reasonable price.
As with all aspects of videomaking, it's a question of taste. Some people prefer the sound of Schoeps microphones, others the Sennheiser MKH range. In the budget section, some swear by the AKG or the K6 range, others are fans of the Audio Technicas. There is no "right" microphone but there are a hell of a lot of "wrong" microphones.
I hope this answers your question "Which cheap microphone should I buy?" and explains why the answer is "None... get something decent."