Oil in the lower cowl can come from a number of sources.
If the oil appears/smells burnt it could be coming from a bad powerhead base gasket or an exhaust cover gasket.
If it's just kinda "goopy" oil residue it could be coming from a fuel leak (gas evaporates and leaves the oil behind), a disconnected bleed line, bad carb gasket, bad intake transfer cover gasket, bad powerhead base gasket, bad upper or lower crank seal, chipped or broken reed and probably a few other things I'm over looking.
You need to try and find the "source" - the result is that mess in the lower cowl.
Once you find "where" it's coming from then you can get a better idea of the cost to repair.
If you can do the work yourself it may be a simple as a 2 dollar gasket - if you need a shop, count on 100 bucks an hour plus parts - a couple hours if you need "them" to track down the issue...