If you want a single bed, Stompa do one that folds out from what looks like a chest of drawers. I think it's called the "bed in a box" (original, huh?).
If you have enough room for a cupboard, you may have enough room for a small chairbed/sofabed. We've just bought one from Ikea for �125. Alternatively, unless you will be having aged parents sleeping on it, why not buy a futon that folds up into a cushion case? So most of the time it's a floor cushion, but can be unrolled to make a sleeping mattress. The Futon Company does them for (I think) about �65 in lots of different colours.