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All your domains are belong to me. 
Seriously, I've been thinking about buying one...just to have. It's only like $50 a year, right? >>
Registration will run about $15 per year. But then to USE it, you gotta have a hosting service. Some registrars offer some basic webspace and email functions, but I've had trouble with the low-cost volume registrar/hosting companies. No service, too much spam.
For hosting, I like
Parcom. They're a family-owned business (you can actually talk with/email the pres of the company), they provide GREAT service, and their basic service is $2.50/month. Advance services are up to $15/month for scripting, etc. I highly recommend them if you want an affordable, all-business/no-nonsense hosting company.