The Marshall BluesBreaker II (BB-2) is around £40-45 and is a 2 function pedal, you can use it as a bluesy OD pedal, or use it as a booster. Selecting the boost function bypasses the gain and tone controls, and only the level pot works, providing around 25dB ish of gain to the front end of your amp. One of the great things about this pedal is it's cheap, another great thing is that it's a tough little pedal, that should withstand some battering about!
I've recently replaced my BB-2 with an MXR Micro Amp, somewhat more expensive (good deals on eBay though), it has a gain knob, and provides about 30dB of gain. It's pretty transparent as well, but there's just something about it I can't put my finger on, that I'm not sure about.