I don't know, next to paying $35/mo for a cheapo cell with unlimited texts and a USB gateway to SMS, you're saying that for moderate users they would need to pay about $9/year to get their internet access on top of that.
Seems reasonable to think that you might pay $5-9/mo on a budget for a limited access to internet such as this; now it sounds like potentially economically viable to even have an employee or two after paying for all the texts.
Assuming the average user gets only one burst of three messages per query, uses it only twice a day for 200 days a year and the service gets only 10,000 users signed up, the costs just to send the texts for the next year alone would be $90,000.
Seems like it will be hard to scale but I agree that this is a really cool way to get connectivity! Best of luck.