You get credited with 10% of your fixed-wing PIC time, up to a maximum of 6 hours (so absolute minimum is 39 hours). You must also sit 1. Aircraft General & Principles of Flight, and 2. Flight Performance & Planning. You must do a minimum of 20 hours Dual.
Certain examiners also deem it unnecessary to undertake the solo x-country qualifier, however I have not seen that written down anywhere. I would suggest checking with the CAA.
I have trained a few fixed-wing converts in various helicopters, professional pilots and high-time recreational flyers. Although they managed it in minimum time it was a squeeze, and they all felt that it was considerably harder (but also more rewarding) than learning fixed-wing.
Costs depend on type, ballpark figures are £200-230 R22, £230-270 H269, £350ish R44 (all ex VAT)