If you suffer from back pain then you might want to consider a memory foam mattress or the Purple Mattress.
Another option would be a Sleep Number Bed and mattress where you can dial in the comfort until it becomes more comfortable for you.
I have back pain and I slept on both a Purple Mattress and a Memory Foam mattress and for me the best mattress for me and my back pain is the Purple Mattress.
The memory foam mattress was pretty comfortable as well but the Purple Mattress is much better so i would highly recommend buying or trying a Purple Brand Mattress because they are very comfortable mattresses and are well worth the money.
But if you need a cheaper mattress then buying a memory foam mattress would be the cheapest alternative.