Specifications:

As Above. It's just smaller, with an improved screen and uses fewer batteries.

Value: £10 - £15

Specifications:

CPU: Custom 8-bit Sharp Z80 variant @ 8MHz
Display: Reflective LCD 160 x 144 160x144. 56 colour display from a pallet of 32,768.
Sound: 2 square waves, 1 programmable 32-sample 4-bit PCM wave, 1 white noise, & 1 audio input from the cartridge. Stereo only when using headphones.
Memory: 32K RAM, 16K Video RAM.
Software Media: Cartridges. Compatible with Game Boy Color.

Value: £10 - £15

Specifications:

CPU: 32-bit ARM7TDMI @ 16.8MHz, with 8-bit Zilog Z80 co-processor.
Display: Reflective LCD 240 x 160. 511 simultaneous colors in character mode and 32,768 simultaneous colors in bitmap mode
Sound: 4 channel CGB compatible (3x square wave, 1x noise), 2 DMA sound channels. Stereo only when using headphones.
Memory: 32K RAM, 256K WRAM (internal CPU), 96K Video RAM (external CPU).
Software Media: Cartridges. Compatible with Original Game Boy & Game Boy Color.

Value: £10 - £20

Specifications:

Internally, as above, but includes backlight (actually a "frontlight".)
Externally, has a vertical form factor with folding clamshell case.
Requires an adaptor to use headphones.

Value: £10 - £20

Specifications:

Internally, as above, but lacking Z80 for the original Game Boy backward compatibility.

Value: £20 - £30