This website is awesome:
http://gun-deals.com/ammo
Not only can you compare prices with most of the top bulk ammo stores, but you can view historical pricing data. You can also find other deals on things like magazines and whatnot. Once you pick the caliber you can narrow it down to the manufacturer, amount of rounds, total price w/ shipping, price per round, etc. It's freakin awesome! I was actually thinking about creating something similar until I came across it...
I just checked it out, cool site
