The US isn't necessarily that much cheaper per flight hour, but it can be considerably cheaper overall due to their more generous licencing requirements.
For example, helicopters are 3-4 times more expensive than airplanes. Which is where cross-crediting comes in.
An EASA CPL(H) requires 185 hours. In the most generous case 135 of those hours must be in a helicopter. (Up to 50 can be airplane time)
An FAA CPL(H) requires 150 hours. There is no minimum requirement for helicopter hours, leaving only the realms of possibility as a limit. Its entirely feasible to get a CPL(H) with less than 50 hours (one guy managed it in 13!)
Now all of that is about getting a CPL as cheaply as possible - so it's technically an entirely accurate answer to you question. If your intention is to fly commercially in Europe then an FAA CPL and 50 hours would be pretty useless. However, for someone planning to stay in the US as a flight instructor while studying for the ATPL exams it could be quite useful as the EASA minimums could be reached in just a few months.