Just answering my own question in case someone else looks for this in the future.
Telcel has a pay-as-you-go 3G modem
plan that is cheaper than what Verizon quoted me for the US!
USB 3G modem
for computer, harware cost = 880 pesos (about USD$70). It's a nice one with a mini-SD card slot and a sim card. The above cost included the sim card.
- activate for a day, week or month at a time by sending a specific text message. The cost for a month of service
is 650 pesos (about USD$50). Unlimited use, but they do throttle the speed back after 3 Gigs of transfer is reached in a month (or prorated amounts in a week, etc). I'm told that does not affect normal use, but it does mean that I can't do online backups.
Works flawlessly and it's fast. I also get service in remote
areas where I wouldn't expect any. But of course the coverage isn't nearly universal.