The ST 8 bits can be $0.283 from ST themselves, but that would be x10,000 (at higher prices, distributors do lower quantities)
"STM8S003F3P6 TSSOP 20 Tube -40 85 Active 0.283 10000"
The 8K flash, 1K Ram, make them very competitive against Microchip/Atmel, I believe. But the case maybe too big for you.
And x10,000 is too many.
Newark (there may be cheaper distributors) gives:
x1000 - x2499 = $0.398
x2500+ = $0.372