Replacing a fixed bed mattress

SWMBO thinks I'm made of money and wants to replace the sprung mattress. I know that it is possible to get a variety of foam replacements with differing degrees of sophistication( memory layers etc) but how easy is it to get a replacement sprung one of the lopped off corner variety and most importantly how much would it cost.

I might threaten her with a bale of straw in the awning as alternative bedding.