The Teensy LC is a US$12 board. About 1/3 the cost of an Arduino Uno. It has a 48MHz ARM Cortex M0+ microcontroller with 62kB Flash and 8kB RAM. EEPROM is emulated in Flash. It is generally a 3.3V board, but there is one 5V digital output, which is created by a 74LV1T125 buffer chip. It has 13 16-bit analog inputs, and one real 12-bit analog output. Seven timers provide about all of the timing one might need.
Unlike it's big brother, the Teensy 3.1, the Teensy LC has two hardware SPI ports, one with a FIFO. There are two I2C ports and an I2S audio port. Under "normal" circumstances, with simple code, the Cortex M0+ keeps up with an M4 at the same clock speed, but the M4 runs circles around the M0+ when it comes to math.
The Arduino libraries supplied by PJRC allow you to write typical Arduino code, but still take advantage of the features of the ARM MCU, such as true analog output.
|PJRC Teensy LC|
|Test Current Draw|
If you need a 3.3V ARM development board with 5V tolerance, try the Teensy 3.1 or Teensy 3.2.