I am trying to build a motor glider and looking for an Engine .
What is the price for an Electric engine?
It depends on details like - Empty Weight, Max. TakeOff Weight, L/D, place of propulsion system (pusher, tractor, retractable), possible diameter of prop...
If it is new design and you have no experience with electric propulsion than I recommend you to start with "known"combustion engine, test real flight characteristics and than install electric propulsion