A great deal of people complete the FAA IR in the US as this allows the student to complete the JAA IR in a 15 hour course.
Although I would always recommend that the student budget for 20 hours back in the land of JAA, a recent quote based on 17 hours, 10 of which in the FNPT II was £4,330. Any additional hours would be charged pro rata.
It is just another way to attain the IR, but you get the two licences for the same price, if not less. Dependant on which direction you are taking, there are many more operators in Europe using US registered aircraft where the FAA IR would be an advantage. Just this month I have seen 4 JAA ATPL's tranferring to FAA for their jobs in the UK.
Good luck with your training.......even though the IR is hard work, I definintely think IR flying is the most rewarding and fun!