If you just need to receive an SMS but don’t actually need a cellular network, Google Voice is free.

If you can stomach $3/mo + tax (totaling a little over $40/year), UltraMobile PayGo is the cheapest consumer option I’ve seen. It’s a T-Mobile owned MVNO. It comes with 100MB, 100 minutes and 100 text messages per month, and is pay as you go after that.

Past that, I’d look into IoT cellular options.