it is not a standard sales trick here but could be in the sates. some companys will offer diffrent quality of bases - platform top, open coils, pocket sprung, and these will all affect the price. some charge diffrent for diffrent qualitys of fabric and most will charge diffrent for drawers or not even leg choice, i have 1 company i deal with that has 7 diffrent legs (well from high quailty wooden one with a brass glide caster to a down and dirty plastic caster) and every one is a diffrent price!
it could just be a case of a 'lower level' one uses less quantity of material to make so is cheeper. what kind of diffrence in price are we talking here?
and a good quality box spring will make a world of diffrence to how you feel on the bed and the lifespan of your matt!
MY ADVICE THAT NEVER CHANGESNo, you dont NEED a firm mattress.
Whatever kind of mattress you choose, if its really cheap it WILL be crap.
Buying a mattress topper just proves you didnt buy the right mattress.
If you have bought a mattress without trying it ... good luck!