A domain name itself shouldn’t cost much if there is nothing behind it. Traffic, reputation- that is a different story!
Imagine, how much effort, energy and time one needs to put into a web project to make it big! By this time it is not just an empty domain, but a web site.
Even these days you can find nice .com and .net domain names at whois.sc for under $10. I would not buy any name above that price (unless it is Google or Yahoo!).