The repair cost will vary greatly as it depends on how severe and where the engine oil leak is coming from. As example; one of the least expensive repairsmay be the; engine valve cover gasket = part is around $ 10.00 on up. The labor may be as high as $100.00 to replace - but again it depends on the leak source, what needs replacing, and the individual repair shop.
There really is no average price or fair price involved as there is price variations even within the same locality. You must be proactive - first diagnose the leak source, then shop for the best repair price - what your budget will allow.
The only 'fair price' often is when you can complete the repairs yourself - at least then you have somewhat obtained a fixed price.
Try the following steps first; thoroughly clean (pressure wash) the engine and all areas you can visibly see any oil leak or evidence. Next, add a small amount of compatible dye into the engine oil. Make sure the engine oil level is filled correctly. Then drive for a time - like normal - and observe if the engine oil leak is made more visible.
This will help to narrow the leak source possibilities and save repair costs. Leaks can be difficult to diagnose and expensive to repair. Half the battle is knowing first the origin of the leak. Try the 'How-To-Guides' feature on this website to get an idea of the leak detection and associated repairs
You may also try the many on-line auto parts sources, such as; www.RockAuto.com or www.autopartswarehouse.com for the replacement parts.