What's fair in a lease back situation?

[Kelly Bellini]

We're in a situation where we're trying to buy a SFR (we plan to turn the walk-out basement into a separate apartment to rent out, while we live upstairs). The sellers want to lease back, while they find a home to move into, and we want to rent it to them, since we realistically don't want to move for another few months. So, it could work out beautifully! But here's the thing: They purchased the home 10 years ago for 180K, and we are now purchasing it for 370K. Obviously, their mortgage payments have been quite low, but our PITI payments will be roughly $2100/mo. Is it unreasonable to ask for that full $2100? I mean, really, we could just get another renter to pay that full amount, or we could just move in earlier than we'd like. The sellers asked if $1500 would be ok?! That seems insane to me. Why would we do that? We are new to the idea of lease backs, so can you let me know if I'm missing something here... What would you do? Thank you!!!!