A personal CREST membership will cost whatever your stock broker charges and the fees for this are very variable. However, the actual fee that Euroclear charges the broker per sponsored member is £10 per year.
The lowest fee for an online stock broker seems to be Charles Stanley’s Fastrade service, which simply passes that cost on. The cheapest overall may be Redmayne Bentley, which waives its £40 yearly fee if you trade during the period.
Obviously, the fee for CREST sponsorship is just a small part of the broker’s charges and you should also consider trading commissions, fees for corporate actions and other costs. See the list of brokers that offer personal CREST account sponsorship for more details.