transmission fluid leaking what to do to fix the problem
Find the bad seal or line and replace it.
how much would it cost to repair
No one can answer your question because no one knows what gasket or seal is leaking and even if anyone did it would only be a rough, or very rough, approximation due to variations in labor charges, etc.
Worst scenario would be a front pump seal which would mean the transmission has to come out. Less severe would be an axle shaft seal, etc.