Hey Geocritter,
 NO, it's not "usual" for a little leak in ANY seal, on todays automotive equipment. Todays (as in the last several years) seals, are much better chemically constructed to withstand thousands and thousands of trouble free miles. If your seal is leaking, IT'S LEAKING period. If you've put too much oil in  the rear diff, it would have leaked pretty close to right after you did so. 
You don't say whether YOU replaced your pinion seal or, had it replaced. But, in any case, replacing an inner seal on any Dana diff, especially a full or, semi-floating one, is CAKE. If you've done any automotive type work at all, you're ok with doing this job. 
The seal can run anywhere from $10 to $50 depending on what axle you have and all that. I did mine, on the drivers side of our '04 Itasca Horizon 36GD with the C-7 330HP CAT and, it took me about 3/4 of day which, included a bunch of cleaning of parts. Yours should about the same, if not easier. 
If you're not sure of the process of how it's done, surely PM me and I'd be happy to walk you through it. Good luck.
Scott