Man....
I was joking.
Domains from a registrar cost from around $6.95 through maybe $35.00 depending on the registrar and what services they offer. Directnic (as Newton stated) is a good choice of registrar. They provide good service, cost $15.00 for a .com or .net registration and have an easy to use front end.
If you want a domain that already exists, then you have to dicker with the folks that own it and then the price will be whatever they decide it is.
If you are just getting started, then like Colin said, your emphasis is on building sites and traffic, not on building a brand. So you go grab a domain name (if it is gonna have adult stuff get a name that sounds adult) There are still plenty of pretty good domain names out there, you just got to be a little creative.
As far as search engines go, the domain name can play a key in how they position your site. IT has been my experience that a keyword rich domain name gets positioned better than a non keyworded name. However, that can give a longer domain name which takes it out of the ranks of a "type in". Type ins are preferred for direct surfer traffic because they are easy to type without making a typo. From what I know, a type in is considred to be 5 characters or less long, but good type ins are hard to come up with these days...
Like I said though, a little creative thinking can get you a good usable domain name that can be used as you learn the ropes and start building traffic and sales. Once you have that going, then you can worry about brand building and all that stuff.
Just my two cents....