Here is a list of the registrars offering cheap domain registration / renewals / transfer

Note:
* Some registrars required a coupon code to have price cut, all coupon code will display in blod.

Yahoo domains
Price: $2.99( $34.95 renewals fees )
# Valid for new customers and first year only.

Mydomain.com
Price: As low as $5.95, depends how many you want to purchase.
Mydomain.com coupon code: htmli.info - 15% OFF of your entire order by using this coupon.
# Valid on all service provided by mydomain.com and all TLD / cctld domain registration, transfer and renewals.

New .NET domain registration
coupon code: webmastersave
Price: $6.25

Register.com
Price: Free! ( You must have a valid credit card )
# One credit card one domain ONLY. $35/year renewals fee

Netfirms.com
Price: $5.95 ( $9.99 renewals fees )
# One customer are eligible to get maximum 5 domains on $5.95, no push function is available.

Godaddy.com
Price: $8.15 ( .com )
Coupon code: cjc795dom

Domain.com
Price: $7.60
Coupon code: htmli.info
# Price valid on new registration, renewals, and transfer. Lower rate if you register in bulk.

Dynadot.com
Price: $2.99 (.info)
# Great if you plan to get a .info domain and they allow test domain too

Namecheap.com
Coupon code: back2school
New registration domain for $8.41 (not valid for renewals)

EuroDNS.com
Coupon code: GOMOBI9
Register and Renew .Mobi domain name now for only 9 Euros.

Hope this help. Reply thread if you found more cheaper price domain.