There are some good special deals going on right now.
Netfirms.com $4.95 for a .com/.net/.org and some cctlds. Do not go directly to Netfirms but instead Google, domain names. Click the Netfirms ad and you will be able to register for $4.95.
Go Daddy is offering $5.99 .com for up to 3 domains. Again Google domain names and click on the Go Daddy Ad.
1and1.com which is not a favorite among domainers, seems to be offering .99 .com for one year. There does not look like there is any other purchase required but do your research before you use them.
Here is a look at standard pricing for a lot of top registrars:
DomainMonster.com $10.99 if you register 25 the price is $7.69
GoDaddy.com $11.99 of course no one pays this price with a multitude of coupon codes usually making the price around $7.69 to $8.
Fabulous.com $9.49 This is at tier 7 with 0 to 100 domains. The cheapest is $7.99 if you have 5000 domains with Fabulous.
Of course there are coupon codes and other deals going on all the time.