If a number plate has ever been purchased from a DVLA auction, then you can find out the 'Date of Purchase' and the 'Price Achieved' by the number plate at that time, by following the link to the DVLA website below...

http://dvlaregistrations.direct.gov.uk/home/en/Search/auction

This information can be particularly useful when valuing a number plate, deciding whether a number plate is worth the asking price, or even looking at data for investing in number plates for profit, although do be aware that it only works for number plates that have been sold at auction.