There always is a cost. That proprietary engine needs to be valuated at fair market price. There is dev cost, depreciation, opportunity costs. And Mole who hates us going on about accounting. looks over shoulder
Seems to be readily available, old school Planeshift has changed from proprietary engine to UE5, and if you are going to look for a niche MMO with small player base and little marketing beyond word of mouth you can't look past that as the prime example;
PlaneShift - A 3D Fantasy MMORPG
Planeshift is a Free Massive Multiplayer Online Role Playing Game, completely made and maintained by volunteers for fun and research. PlaneShift uses an open development approach accepting contributions from the community from code to art.
www.planeshift.it
Old game engine versus new game engine, UE5;
Go the the webpage for the large image, to big to post here and not going to bother resizing.