utwx, provided you don't get stuck with some con artist in my experience of using JAA145 types, the cost are much of a muchness, especially if you compare the hourly rate with say your local Ford dealer. The real pain in the arse is the stupid price of parts and even worse the lenght of time it takes to get even the simplest of bits. The general rule seems to be that apart from routine stuff which you should get a fixed quote for, is that everything takes twice as long and costs twice as much as expected.
Do not expect aircraft engineering outfits to operate to anything like the same servce standards as say your Ford garage. They are in the main under resourced, casual and prone to think the stuff they do is rocket science, and that you as a mere customer are an irritant. If you find someone who will go the extra mile to keep you in the air at that crucial moment when a widget breaks just before you are about to depart for a long planned trip, don't argue about costs, just pay it and keep him sweet. Service and availbility is far far far more important than saving a few quid.