I'd say the value it adds really depends on the house itself.
If it's an older home with some character to begin with, hardwoods can add even more.
Difficult to put a dollar value on it - but it can certainly get more people interested in it, and remembering it as they shop around.
I'd say it's worth it if the house looks like a hardwood floor kind of house - if that makes sense.
Just makes sure whoever does it knows what they're doing.
I'm far from a pro, but have done a dozen or so floors myself. It's hard work to do it right.
I've seen pros do some amazing, glass flat sanding jobs, and I've seen others do what I'd personally consider a failure, and try to pass it of as a job well done.
If you can check out somebody's previous work - take a real close look at the edges of the room. That's an easy area to mess up using a handheld sander. Also look at inside corners. if they're dark, they skimped and didn't scrape/sand them out properly.
And as somebody else pointed out - you could be in for a rude awakening when you rip up the carpet and find a bunch of dog/cat pee stains in the wood.
If they had pets, they have stains. Almost a sure thing.
Sometimes they sand out - sometimes not so much.
finally - look closely at your baseboard molding. People do odd things when they put in carpet. You may find you need new molding - or will need shoe molding after the floor refinish - another cost if you don't do it yourself.