You can check the back support characteristics of any cheap foam mattress by lying on it. It only takes a couple seconds to know if it will be a back breaker or not by checking it first before getting it home.
The reason I say that is because cheaper spring mattresses can be so poor as to not give you any back support, especially in the lower back area where you so desperately need it.
The "6 Inch Polypedic" is the rare cheap mattress model that offers both comfort and support at a low cost. It features a quilted cover and feels like one of the better futon mattresses out there. This is also a great option for bunk beds or any other kind of bed that requires a mattress that isn't too thick. Symbol Mattress manufactures this model in the USA and it is made from CertiPUR-US foams. Perfect for kids or for a short-term solution for adults.
Looking for a Cheap Foam Mattress?
I think you're better off buying a cheap foam mattress, in most cases, than buying a cheap spring or air mattress.
While you're in the store, lie down on your back on the cheap foam mattress and then try sliding your hand between the mattress and the small of your back. Don’t rise up or turn your body in any way to make it easier, just lie flat on your back when doing this little experiment.