I would argue that the main factor in setting rents is the current market rent in surrounding, comparable homes.
If an RSL is given £100k grant funding for a home which costs £300k please explain to me how a private landlord will be able to pay £300k for the property and charge the same rent. I would love to hear how an RSLs extra managment costs, over and above what a private landlord would pay, would exceed £100k for one home! How many years would it take for the PSL to catch up with this £100k if they were charging social rent?
The extra money PSLs have to find will also be attracting interest which they have to cover.