Another quick-and-easy source of RELATIVE values is comicbookrealm.com. The actual prices they give most visibly are for books in 9.4 (although you can easily get their prices in other grades), but the real reason to use it is just to look at the entire run of a title to see which issues are the most valuable, as when there is a 1st appearance of a character deep into a run. Also, I don't know how long it takes to update to reflect what happens when a character is announced to be in a movie, or there is a rumor of such. But it gives you a starting point.