All areas differ of course. Great Question however....
In my area a one bedroom can start at $209k and go to $278k in general. Whereas, a 2 bedroom will start at $289k and can go up.... to about $418. [I just did a quick MLS search to make sure]
So it seems the added value can be from $65,000 and up depending on view, location and overall desireablitiy naturally. Higher values depend a lot also on square feet of improvement.
I hope this has in some way helped.
-Don / Associate Broker