This may be a legacy plan; the only thing they seem to currently offer is $40/line/month for 4 lines ($70/month for one), but throttled at 50GB/month (not clear if this is by line or account), not 2GB/month.

It's a little cheaper than Fi list price at the level Bill Protection (essentially, unlimited with 15GB/line throttling threshold) kicks in ($160/4 lines with TMo vs $205 for Fi). But Fi does refund if you use less than 14 GB total, which may reduce the effective price depending on usage pattern.