There's cheap and there's cheap. The MiniMo 8-bit modular synthesizer at less than €25 is cheap, but it's capable and if that price seems too much to bear, you can get it as a DIY kit (soldering required) for just €14.95.
MiniMo are based on the ATtiny85 processor, a relative of the Arduino. "All miniMOs are identical; you load them with different programs according to the system you want to make. If you have three modules, you can program them to be three oscillators, or two oscillators and a filter, or a sequencer, an oscillator and an envelope generator, or any other configuration that suits your fancy."
- ATtiny85 processor with 8K flash, 512 bytes of SRAM and 512 bytes of EEPROM
- Runs at 8MHz (configurable)
- Powered with cell battery or an external source, from 3 up to 5V
- Four configurable external I/O ports
- Programmable via ISP, compatible with Arduino IDE
- Size: 38x40x30.5 mm (assembled)
- RoHS PCB and components/Lead-free solder (when ordered assembled)
- Mounting holes for legs or covers (or a keychain!)