A couple of weeks ago I've noticed vibrations while driving my '92 Maxima. I checked the spark plugs and one of them was covered with oil on the inside. The compression ratios checked out fine. I've changed the spark plug but it keeps on vibrating. I assume it's vibrating because the oil puts out the spark generated by the plug so one piston ends up not firing. I would really appreciate if someone can tell me the possible causes for the leak and maybe the price range of the fix.

Hmmmmm...

I'd say your FIRST order of business should be a trip to a local Nissan dealer - Drop $100 on a full Consult evaluation and post the results here. Then we can get a better idea of how to help.