Cost is irrelavent, there are too many ways to make it cheap. What you need to be sure of is that you are *really* getting the domain, not just letting them buy it and lease it to you yearly. Find out if you are gonna be the administrative contact, or them. You should also make sure you can move it to another registrar whenever you want, and what they are gonna charge you to do it. Me, I'd stick with big names, I've seen the horror that is a registrar simply going out of business... You call to get your domain moved and get some sick sounding guy on an answering machine...
Big registrars will move them anyway if there is no response from the previous, but it still is a scary thing. A domain name is an asset. Also, if I had the extra cash I wouldn't do it year-by-year and risk forgeting it. Right now there is an insane fad for lowbrow, bullshit companies to crawl registrar databases and grab all the domains the instant they expire, just so they can and point them at low-end, ad-laden search engines. Worthy of the Capital Punishment? very, very close call...