There is no clear answer.

That is like asking what does a Ford Mustang cost.
Every project is different, so no two projects are going to be the same price.

It can be anywhere between free, and $30 a sq.ft. or more.
Your perceived value, may not meet your expectations.

Real good answer!

As far as installers in the bay area, I would check Angie's List and then check references of the installers you are considering.

Angie's list is not a very good indicator.
You have to pay to post there.
Contractors using multiple e-mail addresses, give themselves good ratings.

I've gotten 3 quotes and they vary from $4.45 to $5.80 per square foot for installation only (not including the wood). The highest quote did not include prepping the subfloor, which we estimated at an additional $500. I'm installing 800 square feet of prefinished 5" red oak planks, solid 3/4" hardwood, over slightly uneven old douglas fir subfloor in a 100 year old home.
I have had great luck with finding vendors on Angie's list, and decent luck with Yelp.