Got a 'Dunlopillo' matress which is well comfy and has kept it's shape and not not sagged at all in 2 years, cost us around £600 for a standard double size.
Do not get an IKEA matress they are crap, even the most expensive one.
If you can afford foam it's the dogs. My ex did it on the cheap yesterday and went to the foam store in Brighton where she got a double bed-sized piece cut to her specs for £100 all in.
Spend as much as you can afford. I invested in a Dunlopillo foam one a few years ago and it is still going well (or so I am told!). Foam mattresses do tend to be warmer than conventional ones, so if you don't like being too warm, it may not be for you.