We'd need to know a bit more about the leak and where it is. For instance it might be from the driveshaft seals or the gearchange selector shaft or the first motion shaft (input shaft) or the end cover gasket or ? Cost will vary enormously depending on where the leak is coming from.
Gearboxes, regardless of make of vehicle, do tend to leak a little bit, our Panda has a leak from it's gear selector shaft at this time and has been gently "weeping" for a couple of years now. Last year I fixed the same leak on my boy's Punto as it was becoming rather more than a "weep".
If it's only a small leak - a "dampness" rather than an actual drip of oil and not leaving an oil mark on the ground where you normally park up - I would tend to leave well alone whilst checking on it maybe every couple of months to be sure it's getting no worse. I would also check the gearbox oil level every year at service time and if it needed more than just a spoonful of oil to top up I'd take it as an indication it was time to sort the leak. One leak I wouldn't delay fixing would be if there was any chance the oil might contaminate the clutch, then I would do it a.s.a.p.
If you could post a picture of where the oil is leaking from that would be very helpful.