I have never had a cheaper one. When we bought ours, they were much less common and the difference in price between tempur and the others was much less so we just went straight for it.
I'd seriously suggest you look at Which? before buying a Tempur mattress...it's the only "Don't Buy" brand they tested. They said that they are wildly overpriced and have poor durability...there are far superior memory foam mattresses available at a third of Tempur prices.
I had a tempur brand for about 13 years and it still seems fine. My woman has a cheaper thicker overlay on her bed. It's softer than my mattress but seems fine also. Maybe best to try stuff out in the showroom ? Not that I did; I bought on the Net based on tempur reputation.