You don't actually need an FAA medical to fly in the US if you have a JAA medical....that is if your FAA licence is issued on the basis of your JAA licence.
"Holds a current medical certificate issued under part 67 of this chapter or a current medical certificate issued by the country that issued the person's foreign pilot license;"
Initial JAA Class 2 around 120 GBP, as you have all the extra crap like ECG. When I had it, the AME was so embarrased at the cost, he gave me the FAA one for free:-)