That sounds like a good price... I can't remember off the top of my head, but I think the local shops stock boxes of 12 gauge for about $7. I can't remember how many shells though... Going by Cerendrad's post, probably 20 shells to a box.
12 gauge shells tend to be relatively cheap... the issue is if you start getting into less common gauges (10, 16, 20, .410), or into certain rifle calibers (which doesn't really apply here).