It's a bit more than that - probably 40% difference these days. In JAA land the difference was literally the IR subjects - the old CPL has now become the ATPL(H) VFR and they created a new, lower standard CPL exam. Hence repeating 12 exams again.
Given that you can be a captain on quite large helicopters as a CPL holder, the only reason to do an ATP would be to have it on your resume when looking at the offfshore market. You would also have to do an MCC course and do a type rating on a machine that requires 2 pilots, which can only effectively be done for most people through a large company.