1998 Nissan Sentra Front CAM seal troubleshooting

I have a 1998 Nissan Sentra with nearly 260,000 miles on it. Recently it developed an oil leak on the left side(facing the front of the car) of the engine, at least I think it's coming from the engine. Everything mounted in the area has a thin coat of oil on it. I took it to a garage and was told that it most likely was a front cam seal gone bad. My question is, how big of a repair job is a front cam seal replacement, and if you could give me a ballpark fiqure on what it most likely would cost? My concern is the number of miles on the car and if it turns out to be a costly repair, is it worth it. Will the leak worsen?