You can buy a Raspberry Pi Model A+ right now. It costs around $20/£15, it’s just 56mm long, 12mm thick and uses up to 45% less power than a Model B+…

Model A+ Specs:

Dimensions: 65x56x12mm
Memory: 256MB RAM
Expansion: Micro SD slot,
1x USB, 40x GPIO
Networking: None
Price: ~$20/£15