Depends who you buy it from I suppose. The average price seems to be around $6.00 and upwards. As for your other question though, that really kind of depends. In a sense it might because most Web newbies assume most (or even all) domains are .com and could perhaps be more easily accessed by them. However, in terms of search engine rankings it wouldn't matter what you go with. What it really depends on is the quality of your content and your markup (i.e., semantics) and other such things as who is linking to your pages.
I got my domain name for $15.00 a year...through www.directnic.com but their features are a little limited I believe...like the php support and such...I don't think they offer that on their hosting... but I could be wrong..last I knew they were talking about making some changes..But Im sure there is more out there for a cheaper price...
.com would proberly get more visitors, but it depends purely on who will be visiting it and how, for example if I was to flash a link up on every post people would proberly use the link more than the url so it wouldn't matter. using .com is great for noobs using a computer since windows has a ctr + enter trick which turns: "hello" into "http://www.hello.com" instantly, while I aslo know a rather slow person(not because he cannot use a computer, he generaly is very slow, he seeks all kind of special needs to read and everything) who types .com at the end of everything on IE, even folder names and file names.
i use omnis its about $8 USD and reliable and easy to use, so no complaints.
as far as .com over the rest? from what i understand its best to always get a .com,
an example of why a .com is better:
if your site is something.net and your competor is something.com, you dont want potental customers to end up at your competator by mistake.