If you have leaking fork seals, you must change them and add new fork oil—it’s part of the regular maintenance of a motorcycle. The leaking oil can ruin your brake pads and after the oil drains you can damage your fork. The seals should be replaced at the first sign of leaking oil. Replacing fork seals in a shop can cost about $150-$300 or more depending on your motorcycle. You can save on labor rates (about 30 minutes) if you detach the fork tubes and take them to the shop.
If you are mechanically inclined and have the right seal driver, you should be able to do it yourself. A seal driver, plus the new seals and new fork oil can cost you about $100. Remember to consult your manual. The process varies from motorcycle to another and is not for beginners.
Now watch this popular video tutorial on changing forks seals: