The DVLA have some plates up at a fixed price, and then they alternate between a live and a timed auction - although at the moment the next live auction has been switched to a timed one. Both types of auctions have reserve prices, and you can't get a plate from the DVLA for less than the reserve. Bidding at the last timed auction, which ended last wednesday, was very light - quite a few plates had no bids at all and many plates sold for "just" the reserve. If nothing else, looking at the DVLA auction reserve prices and sale prices will give you a feel for what people consider these plates to be worth - obviously there are massive variations, but you start to see patterns like anything with an X or a Z tends to be a lot cheaper.
If I were looking for a plate, I'd first check if I could get one I wanted at a fixed price from DVLA, if not then I'd look on ebay (but not pay inflated prices). Failing that I'd start to check the listings for the DVLA auctions to see if there was anything I wanted coming up at auction. Only if all of the above failed, and I'd run out of patience, would I start to look at the companies that sell them.