I just found this website that looked somewhat helpful:

Mobile Roaming advice | Compare mobile roaming costs with RoamingSims.com and save up to 85% on roaming charges

Heads up: I haven't given it a deep read. The website is focused on Brits. All prices are in GBPs and they compare rates against UK carriers, but it is still helpful. I still prefer prepaidgsm.net, but they definitely have a place.