Have you heard of castile soap? It's liquid, all-natural soap (extra virgin coconut, olive, jojoba and hemp oils) that works wonders. Really, it's very powerful. Dr. Bronner's is the brand I've found, available at Trader Joe's. It may seem a little pricey, but it's so potent that a little goes a long way. Also, I recommend buying a big bottle of vinegar and a large box of baking soda. The more you buy, the cheaper the unit cost.
I'm convinced that most commercial cleaning products out there are simply a marketing scheme. Before Soft Scrub, how did anyone clean their tubs? That's what they want you to think, but a few generations ago they used common household products to clean. Where did that knowledge go?
Tub Cleaner
from Clean House, Clean Planet by Karen Logan
1 2/3 c. baking soda
1/2 c. liquid castile soap
2 Tbsp. water
2 Tbsp. white vinegar
Mix baking soda with liquid soap in a bowl. Add water. Mix with a fork until smooth. Stir in vinegar. Scoop the paste into a pint-size glass jar. Add warm water if it dries out before you use it all, and stir before use if needed.