Can a mortise replacement cost $250?

I would recommend several things. First thing I would do is to reinstall the lock you have now and make sure it is properly adjusted. Locks will fail when undue stress is placed on them. The latch should engage and hold the door without undue pressure, and the bolt should fully throw without any drag.

Now start looking for a replacement lock. I would first contact the manufacturer of your doors with a good photo copy of the lock that you add the dimensions. Keep this as a spare. If you have trouble with this process, call back the locksmith that was able to fix your lock, seems to me he has earned your future business.

I would not press too hard on retrofitting the door with anther kind of lock that is radically different from what you have now. There is a reason you have this kind of lock, probably the width of your style.

Good luck.