Official Arduino boards are pretty cheap, but another option is to assemble your own with a few cheap components and a breadboard. Of course the great thing about a breadboard Arduino compatible, apart from saving you a few quid, is that you can tinker with the board itself, as well as the stuff you connect to it.
There’s a visual guide to creating one from scratch on the Arduino site:
And if you don’t want to hunt around or buy all the components separately there are kits of parts available. Maker Shed have one called the MintDuino:
for which there’s also a book:
If you’re in the UK and you don’t want to pay the international mailing charges on the MintDuino, oomlout also have neat little kit:
This one looks more complex than it actually is because they use 0 ohm resistors as jump wires. One other word of warning about this kit – in the version I received there was a minor difference between the circuit shown in the assembly guide and the layout shown on the breadboard overlay. That aside, it’s pretty simple to build and you can have a working prototyping board in 30 minutes. Here’s a timelapse video of mine being assembled.