I'm not sure what all the talk about return ratios and profit is--you simply left a message that you wanted to keep star tortoises. If you are looking for that sort of thing, profits and such, this may not be the right species. A hatchling female will not be at the height of producing for a decade, and there is plenty that can happen to this relatively fragile species in the interim.
If you're less concerned about profits and such, and have the time and energy for what is probably the most demanding popular tort to keep, then you'll enjoy them.
I've raised many pair from hatchlings, and if done at the right pace, females are mature in about 8 years, and males in about 5.
There is no real difference between Sri's and mainlands, and they are interbreedable. I have seen move than a couple Southern Indians that put the average Sri Lankan to shame.
They will not eat timothy hay as a staple, but it is a good substrate, so long as yuou keep it clean. As you know, you can find it in the small animal section of any pet store.
A quick online search will get you lots of testimony about star diets. http://www.tortoisetrust.org/articles/elegans.html