If the domain name your after is parked, and its any good, just the fact that you've inquired about it can add value for the current owner. If the .com is fully developed, then arguable the next best alternative for those exact keywords is the .net extension which could indicate that others have also inquired and therefore inflate the asking price..
As far as development costs, you could attempt to negotiate a deduction of all or a portion from the asking price. The fact that their undeveloped actually works in your favor as it could theoretically undermine the value in the eyes of the current owner.
Short of iron clad revenue stats, the domains are being sold on a combination of development potential and generic value, both of which hold different meanings for different end-users.
If the owner is selling for a perceived revenue multiple based on search volume, try to beat up the math a little bit, come up with a realistic estimate to debunk theirs, deduct costs of development and counteroffer accordingly. Then hurry up and wait..
I also negotiate "on behalf of my client". It tends to level the playing field a bit as far as price expectation.