i cant really qoute a price without seeing it. it may be something simple, or it may be major. either way, i'd probably be way off. something you may want to consider is;
go over to the mra and look up tabpat. he is in waco texas. he may be able to look at it for you. it would save you some shipping cost. as for me, i'm so backed up in lawn mower service it might be 3 weeks from the time i recieve it till i can look at it. if i lived down there, i'd look at it for you for nothing, and then i'd be better able to qoute a repair price.
since you have the transmission cover off, remove the gear that rides on the crank to drive the transmission. that will isolate the problem to either the crankshaft, or transmission.