Most of us don't have money to go to the spa, but we have time. If we were spending more of that time making money we could go to the spa, but who wants to do that?
There are really only three steps to making a serious load of combinations as far as bath salts go. Oil, sugar/salt and essential oil. With just those three ingredients you can mix and match a number of luxurious scrubs that often come out way better than anything you'd pay a reasonable amount for at the store.
Here are just a few examples:
See how easy that recipe is. Get creative it's just Sugar/Salt + Oil + scented oil. You can throw in herbs as well if you want to step it up, you know dress it up a little.