Is it hard to fix a leak coming from my transfer case and can it be expensive?

Not a huge thing to have removed and new seals installed. If its the output shaft seal leaking it would not need to be removed to repair. Probaly going to be cheaper to have repaired rather than waiting till it gets low on oil and it totaly fails.

if you remove it its approx 225.00 to reseal the whole case, if you want it done its approx 500.00 job parts and labor

If you don't fix the leak it will cost you a whole lot more for transfer case.