I doubt it is cheaper at all.
H-1B applications must pass the prevailing wage test, i.e. applicants cannot earn less than the prevailing wage for their position at the chosen employment location.
Companies like Tata, Infosys, WiPro, etc. need to maintain a part of their workforce in the US for certain projects anyway, so they slave away these applicants with draconian contracts. It's not that they pay them significantly less than any other, but the fact that they can guarantee a 3 to 5 year headcount for their larger US based projects, with a larger talent pool to choose from, if needed.