The overall reason that sleeping on a good mattress every night is that it's good for your health. If it costs a little extra money to get something that's going to give you a better night's sleep, and more energy the next day, and better overall health to boot, you should at least figure out a way to be able to afford a good mattress. You may have to save up an extra month or quit going out to eat as much (which will be good for your health too!), but if you put your mind to it, there's no excuse for not being able to have the money to spend on a good bed.
If you look at your budget and how much you spend on certain things, you might be surprised. When you take your health into account, though, it's worth any money spent to help maintain it. A good night's sleep can go a long way toward a happier and healthier you. There are, of course, many other reasons that it's important to sleep on a good mattress, but you can't argue with your overall health being a good enough reason to invest the money in a good night's sleep.