Texas Instruments practically gives away their development boards. Here is their Stellaris Launchpad board with ARM Cortex-M4F-based MCU for just 8$! Much cheaper than even AVR arduinos!
https://estore.ti.com/Stellaris-LaunchPad.aspxWhat's more, these days you can even use the Arduino IDE for development on these boards. The Energia project has ported Wiring and Arduino IDE for TI boards. If I wasn't such a minimalist, I would get one (or two) in a heartbeat!
Edited: Forgot to paste the link.