I think US$15 is the ordinary registration fee. Depends which service you use, though. Also, you really shouldn't use the .com top-level domain unless you're actually running a commercial site. I know everybody does but that's not what it's for.
You have to pay. Just go to domain.com or something. The standard price is $15 to buy. A domain name will do nothing for you unless you're renting a server that you can link it to. It's just a link, a server costs a ton though, so I suggest finding a free hosting site, buying the domain, and linking it to your homepage on that site. If you have comcast high-speed, I think they give you a homepage.
Doteasy offers .COM sites with 100mb space and 1gb yearly data transfer (and a whole bunch of other technical gobbelygook) for 25$ USD a year. That's what I use.
EDIT: Whoops, forgot to put a link to doteasy's site in. http://www.doteasy.com it's not too hard to figure out. Oh, and one more thing, 25$ a year USD is the cost of the domain name, Doteasy offers the space for free! Because domain names cost 25$ a year, you will not find any site that gives you one for less than that.