What should I be paying for average, good and excellent quality DV Tapes? Are there any grades of quality that should be recommended?

Last ones I bought were these. Amazon.co.uk: Sony 60min Premium MiniDV Tape - 5 Pack: Electronics & Photo: Sony
I use these to record HDV and not had any problems with any of them in two and a half years.

I will add this though... This may prove to be historic but it was certainly the case a while ago. Better to choose a brand and stick with it. Different manufacturers have used different type of lube before now and they have proved tobe incompatible with each other on occasion and can clog up your tape mechanism. The genaral rule here was use Sony or use any of the others but don't mix.

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I'm not young enough to know everything!