what should it cost to repair a leaking front differential?

sounds like an axle seal leaking and the cost should be in a guide that is on this site.

Really depends where the leak is coming from, is it a transaxle seal, a pan gasket seal etc?

Unfortunately, front trans-axle / differential leaks on Toyota vehicles are common. Issue stems from both transmission half's being clam shelled around the transmission internals. The issue could also be an oil leak from somewhere above the trans-axle unit. You may want to consider a second opinion before taking your vehicle under the "knife".