Try and observe the leak. If you can't visually notice a leak but you're losing oil, an internal engine or other problem may be present, or your engine may be "burning" too much oil. Usually leaks occur when a seal has gone bad, so more often than not you'll be needing a seal replacement somewhere. There is all kinds of seals inside the engine such as the rear main and valve cover gasket. The cost to replace depends on seal location. For example, don't be surprised if you go to a shop and are quoted around $300 for a valve cover gasket. It may be a $10 part, but the labor to do it is significant. In summary, try and locate the leak and ID the seal in question. If you can't see the leak, you can take your vehicle to a shop and for a nominal fee (shouldn't exceed one hour of labor) have a diagnostic done. If you have the know how you can fix the problem yourself, or have the shop do it for you. Furthermore, park your vehicle over some white paper (such as butcher paper), or newspaper. Park it overnight and on the next day go and see if any liquid fell onto the paper, if so, then the problem is most likely external. Also, try to note straight up (vertically) from the spot to ID the leak, as gravity tends to pull liquids down in a straight line. Rain, snow, or windy weather will affect your test results, so make sure you do this in the driest conditions possible. Hope this helps.