Using Prepaid SIM card in MEXICO?

Hi, I'm a student who is heading to Queretaro, Mexico for an exchange. Can I check what's the cost of prepaid sim card in Mexico and which are the companies that have better coverage? I'm looking to use the card for both domestic(mainly) and international phone call, and preferably, with 3G coverage.

In addition, I'm travelling over to LA and AZ, USA after my 6 weeks in Mexico, can I also check which prepaid SIM card to get? I'm using an iphone 4s.

I'm not sure about using an iPhone 4 because it uses the smaller sim cards. I know you can buy the regular sized sim cards for Telcel's pay as you go Amigo system all over the place including OXXO stores. They may have the small cards, I'm not sure but I just haven't seen them. You may need to go to an actual Telcel office to get the smaller sim card.

Your iPhone 4 would likely need to be unlocked to.

There are other companies but I've only used Telcel so I can't tell you about them. If you went to a larger mall in any medium to large city I would think you would be able to find at least a couple of choices for cell carriers either with actual store front/office space or at least with kiosks.