It depends at what stage you convert as to how much it costs. An FAA PPL is just fine as a precursor to doing a JAA CPL course, so no need to convert it to a JAA PPL.
If you go all the way through to getting an FAA CPL/IR you'll have the costs of the 14 JAA exams, whatever training you need to get upto test standard (CPL is 'as required', IR is minimum of 15hrs), then the 2 tests at about £750 each.
It would be more helpful if you clarified at what stage you wish to convert. Are you really going to America and not coming back until you have an ATP? Are you just doing a PPL and hour building? Are you doing an IR over there? Ask an accurate question and you'll get an accurate(ish) answer.