Replacing the front wheel bearings is a simple job and if you get the bearings, cones and seal at a Kaman Bearing type store, it shouldn't cost more than $25 instead of $200 at the dealer. There should be one (bearing store) in your area, see yellow pages.
There probably isn't a cotter pin in a locknut. MB uses a contracting locknut that is tightened on the threads by an Allen head screw, 8mm I think.
If you keep driving, the bearing could seize on the spindle and you will need a torch to get it off (happened to me).
P E H