I have used GoDaddy for almost all of my domains (about 10) just because of the ease of starting a domain there. I dont know it might be a bias statement since GoDaddy was the registrar for my first domain. I had never had a bad experience with them. Try searching google for the term "Cheapest domain names". Watch out though some companies require you to buy hosting packages for the nice deals.
enom offers only $7.95/yr if you have a reseller account.
I would recommend few more cheapest but reliable Domain Registration companies that are Domainsite, namecheap, namepro.
You can go for anyone out of these.