As Redsnail says if you have a full ATPL and heavy experience it makes life a little easier with the 'conversion' however here is the breakdown for me after I returned to the UK with FAA CPL/IR/ME and 1500 hours:
JAR Theory Distance Learning + Exams : £3000
JAR CPL course + CAA Test : £3000
JAR IR course + CAA Test £12000
MCC Course £2800
Total about 20800
This was with a full set of first time passes - it could get more expensive.
Good Luck