redoing my basement how much ?

[s0nIc]

So basically were letting my wifes brother and his Wife move in .I dont mind They are taking my basement }: so no more media room.Hey its family they asked to stay for 2 years I said fine we talked about bills and so on.Heres the twist my basement has the wood flooring and Drywall done already.BUt the roof and the stairs and they told me if they dont mind us have a pro come out and fix the stairs and put the ceiling on and they would pick up the bill.
I sent a few request for an estimate but my wife feals like we should help out with half synce it is our house.My question to you guys is how much do you think something like this cost before I open my mouth.
Basically they want to close of the stairs and put carpet on it and close off the center pieces becaause of the baby.We would also need a type of ceiling we would be able to get to the pipes.