Most good mattresses ARE pricey, but there are two Which? Best Buys that are not....both by Ikea. They are the Ikea Morgedal and the Ikea Hovag...rated as good as anything costing 3 or 4 times as much.
Happy Beds Contour
we bought one via Amazon for our single spare bed. Used to be our sons but now his daughters are starting to stay over!
It is so comfortable that our daughter slept there and immediately bought a King size (£200) for her house.
It is a mixture of traditional coil and memory foam. For a budget one you wont do better.