to get a baseline price multiply by the number of hours X $10.00 (standard commercial depreciation).In your case this is $5000.00 per engine. Take your initial purchase price minus tax , props and rigging.and minus out the depreciaton , giving your baseline. (if you paid $15000. each , they are now worth $10,000.)
Obviously they're are a lot of variables here , made , maintenance , condition,engine age , and what market you are in but this will give you a starting point.
beemer