Firstly you must understand that there are many stages to the conversion.
- ATPL Groundschool
- JAA CPL
- JAA IR
The first step is to get yourself a JAA class one medical if you don't all ready have one.
The ATPL groundschool if you have no credits is 650 hours and on average takes 6-8 months for someone to complete and there are 14 exams. Normally circa $4,000-$5,000.
The JAA CPL can be completed in the USA and is training as required. Normally on average around 10 hours and around 2 weeks. Normally circa $3,000-$4,000 including exam fees etc for a good student.
The JAA IR conversion must be completed in JAA airspace i.e Europe and is 10 hours sim minimum and 5 hours aircraft. Normally circa $5,000 cheaply for a good student and takes around 2-3 weeks.
If the UK authority add two exam fees which run circa $1,300 each that's $2,600 plus aircraft hire for the test and for a good student at a cheap cost circa. $13,000. I would say a more realistic figure is circa $17,000 for an average student coming out of FAA training.