How hard would it be to fix the shift shaft seal on a dirtbike?

I've got an oil leak at the shifter and I'm not wanting to open up that part of the bike so If I took it to the shop how much would they charge? because I've looked into it and I'm thinkin its the seal inside the engine.

Well, you aren't too specific about what kind of bike, but you may be able to change it yourself if you can take off the shift pedal so only the shaft and the case remain, the seal can probably be changed from outside the case. It will probably fit into a boss where the shaft comes out. You'd have to look at it after you get the pedal or arm off. It may be that you can get a scribe or a very small straight screwdriver, and tap it into the seal with a hammer, and then pry it a little, and do the same on the other side, until you get the seal out. Then get a new seal, and use a deep-well socket (Like a spark plug socket) to push the new one into place. Don't forget to lubricate the seal and shaft before you drive it in. If you don't have a deep well socket, check with a neighbor and see if you can borrow one for a few minutes. You want the socket to bear on the outer edge of the seal, but not protrude outside of the outer diameter, so you don't damage the case.
Good Luck!