The Wal-Mart T-Mobile 5GB data / Unlimited Text pay-as-you-go plan (with a couple of voice minutes thrown in) is $30/mo. However, the contract does not technically allow tethering.

T-Mobile were the last major US carrier to institute a pay-extra-for-tethering policy (in late 2010), but it's definitely in effect now and they do use some cursory heuristics (data usage, basic traffic inspection) to detect tethering (and unauthorized "high-end" Android phones, as well).