Clearly skipped my post about how they're all made the same and are pretty much the same under the cover, a piece of memory foam glued to the top of a piece of firm foam.
My king size cost me £230 (delivered) and is considered a cheapo one despite it having all the exact same quality's as an expensive one, cool sleep, gel foam, anti allergy, anti bacterial, impregnated with aloe vera, removable and washable cover.
I wouldn't have paid more than £300 for a mem foam mattress in king size because they're they're just not worth it, they're ridiculously easy to produce compared to traditional metal coil sprung mattresses (again, 2 pieces of foam cut to size and glued together).
Don't come round here spouting subjective fanboy nonsense bigging up your expensive foam mattresses and knocking cheap ones when the facts have already been stated.
The Breasley website home page even proves my point.
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They're all made the same and are pretty much identical underneath the cover (notice in the Breasley warehouse there's only 4 grades of foam).
Buy the cheapest one you can find that has all the quality's you want.
The biggest aspect is how thick the mem foam top is, the thicker the better and usually more expensive the mattress.
Shop around, don't pay for hype.