Im not on a plan as far as i know !
When i originally opened an account with them these were the prices quoted and have been the same ever since, they may have increased prices slightly i am not sure on exact figures as they do not provide a statement for some reason.
I do not pay more than around £1 for a basic ID check + CAP valuation. Yes it jumps to around £4 for full HPI check, but i have no minimum requirements at all.
Like i said speak to them and see what they can do, The prices you have been quoted seem a bit extortionate.
Maybe some other traders can confirm what deal they are on with HPI...?