I've been doing this for a little while now myself. Mostly through ebay sold listings, but those can be tricky because running the same search different days or times sometimes delivers different results. So I'll get an average sold price, then look up the book again, and I can't even find one of the sold listings I found the previous attempt.
The other thing to be wary of, is that the prices you'll find are entirely based on the condition of the book, and if you haven't scrutinized a comic before, you may be starting with an inflated number compared to your actual book. If they aren't really old, then they need to be just about perfect.
FN-2199 had a good suggestion using the Overstreet price guide just to identify the one book out of a run of 100 that might be worth more than the rest. That price probably won't be the going rate, but at least you'll know which ones to start checking.