Adding WiFi to OSLess embedded device
TI sells the CC3000 that is very much RTOSless. The stack and everything is on the chip/module, and you just need a driver that's about 6k of code, 3k of RAM (sometimes even less if you're willing to sacrifice throughput).

Last I saw, the Evaluation board which is just CC3000 (or booster pack for launch pad) was going for around $30.