A good aged domain with history and pagerank will set you back anything from $50-$300+ depending on how good it is.
Those things taken into consideration for value of a domain name:
.com = 1st .net = 2nd etc..
the value of keywords ie. bestmortgagecompany.com those keywords get a lot of searches so the domain is worth a lot more
lack of hyphens=more value (not best-mortgage-company.com)
number of backlinks and the pagerank of the domain (PR3+ is good)
the age of the domain
whether the domain has been indexed
whether there is any existing traffic (alexa ranking is an indicator of this)
whether a site already exists and is being sold with the domain.
When you buy a domain from scratch, you should put a website on it as soon as possible, so that it registers with google. Trust goes up with time and links, so start as early as possible.
For existing domains, you don't need as many (if any) links to rank in the search engines.