I've made 2 batches, both different but with similar ingredients. Basically they are salt, sugar(brown and/or white), garlic powder, onion powder, paprika, chili powder, cumin, season salt, dry mustard, cinnamon, ground clove and basically anything you've got. I made mine for slow cooking Boston butts but have used it of sashimi grade tuna, chicken and other things. Some folks put ground coffee in it too but I haven't.
Check out a Dollar Store of similar for cheap spices.