Hardwood installation costs

Sorry for double post!!! Had to allow comments….
Hello All!
We need to install hardwood flooring in three rooms of our brownstone. We already have all the supplies and tools (hardwood, molding, etc). Three questions-….
1) Per sq ft, whats the general cost for installation of hardwood flooring on subfloor (which is already there)?
2) Whats the general cost for removal of old wood flooring and installation of new hardwood flooring (subfloor is already there)?
3) What company would you recommend?