Ting (choice of Sprint or T-Mobile) is $6/line, plus usage. Google Fi (uses both Sprint and T-Mobile) allows subscribers to add data-only SIMs to their existing plans at no additional charge (other than data usage).

Check out charge.co. Discussion from a few days ago: https://news.ycombinator.com/item?id=11744875

Looks like this was just announced today from Twilio: https://news.ycombinator.com/item?id=11763067

Could be a great pairing.

In Canada, FWIW, you can get tranched plans from most carriers starting at $5 month (it is their "tablet" option). The bills go up pretty quickly if you use it much, but still a decent option.