[ringworld]

In the USA, the cheapest option I've found is Tello (T-Mobile MVNO) - $5-6/mo for the absolute basic tier, real SIM with a real (non VOIP) number. Accepts number port-in after initial activation.

[projektfu]

If you just need to receive the occasional text message, you can get a free one from FreedomPop.

[bxparks]

Red Pocket has their cheapest plan at $30/year ($2.50/month) on T-Mobile. 200 min, 1000 txt, 200 MB per month. Real SIM card. Available only on eBay. No affliation, just a satisfied customer.