Price: 30$ + 10$ shipping
I think this board would be a great replacement for the arduino, esp in tasks such as ardupilot, arduimu, etc where limitations are present (ie Kalman filters are too slow, ROM is too small, etc). A huge benefit here is also that there are two DACs (not PWMs, real DACs!) and not only that, but they're 12-bit. The ADCs are also 12-bit, compared to the Arduino's 10-bit ones (and there are 16 ADCs vs. 6 on the Arduino).
This board is SMT only which I think is reasonable (and lighter weight as well).
The board features: (from the website)
Bootloader via RS232 (not USB) (which saves weight)
32Bit ARM Cortex-M3 running at 72MHz 64Pin (LQFP) by STmicroelectronics
Use Crystal 8.00MHz + Phase-Locked Loop (PLL) - computes at 1.25 DMIPS/MHz to process data that is equivalent to 90MIPS
Has RTC (Real Time Clock) Circuit with XTAL 32.768KHz and Battery Backup connector
5 Serial ports
48 General purpose input/output pins (GPIO)
16 Analog input channels 12-bits (ADC)
2 Analog output channels 12-bits (DAC)
512 Kbytes of Flash memory
68 Kbytes of RAM
3 SPI ports (2 can do I2S)
Supports In-System Programming (ISP) and In-Application Programming (IAP) through On-Chip Boot-Loader Software via Port USART1 (RS232)
Has RS232 Communication Circuit that uses Connector 4-PIN
Has 48 Bit GPIO with 5V-Tolerant for independent digital applications (maximum analog input is 3.3V)
Use +3.3V Power Supply
Connectors is placed on 50 Pin Header with a distance of 2.54mm (100mil) (25 pins per side) with a distance of 1.5 inch (1500mil/38.1mm), so it is quite easy to use and expand I/O Circuit.