I am lost here, you hold a JAA ATPL? If you hold a JAA ATPL then you just need to renew the SEP rating on it which subject to the standard 12hrs in 2 years and a flight with an Instructor. Where is all the extra cost coming from, as an ATPL you already have a Class 1 so no extra cost there, you intend to fly SEP (otherwise why renew?) so no cost there, BFR under FAA will cost you an hour, 2 year flight with an Instructor will cost you an hour. No extra fees to pay. Your ATPL I assume will be renewed which covers the SEP or are you giving it up?
If you have a restriucted FAA licence already it is based on your CAA reference number. As long as your licence has the same reference number now as it did when you originally did your PPL then all you need to do us a BFR. If you have a new licence number (unusual put possible) then you will just need to apply for a revalidation with the CAA. Your CAA reference number stays constant you just add rating to it, PPL, CPL, ATPL, etc.