That all depends on exactly where the leak is.

 If it's from an output shaft seal, they can be pulled and replaced
in an hour or two by an experienced mechanic, they might have to use
air tools to get the U-joint bolts loose and off.  If it is leaking
from a gasketed side or top cover plate that can be unbolted and
removed with the transfer case still in the car, again it's an hour or
two.

 I can't give you an exact price because I don't know what the seals
or gaskets you need will cost, or if you are in a high-labor-rate area
like New York City, but figure a few hundred.

 But if it's the input shaft seal, or a gasket between sections of
the main body, that can get pricy.  If they have to take the transfer
case all the way out of the car to fix it, that's at least 3 to 4
hours.  Or all weekend at DIY speeds.

 That's the time to decide whether it's leaking bad enough to justify
the repair bill.  If it's only a drop or two a day, I wouldn't worry -
just don't park on someone's brand new driveway if it drips, and
remember to crawl under there and top off the oil as needed.

 But if it's pouring out gear lube to the point where the case is
half empty in a week, then it really does need to be fixed.

     --<< Bruce >>--