Unless there is some medical reason, I don't think you need to spend that much to be honest.
I looked into this about 8 weeks ago when I bought a new mattress. I ended up with a Breasley Pocket 1000 and I couldn't be happier with it. There are more expensive ones, I tried the Pocket 1600 which was dearer and less comfortable. The saleman told me the more expensive version was for people who were heavier, i.e. the cheaper product was more comfortable and more appropriate for me (and the dear lady on her side as well). I'm 70kg by the way.