When buying or renewing a domain name you can pay a variety of prices and with an equal mix of service to go with it.
Our current recommendation is for long term use is Namecheap. The provide a good level of service at reasonable price that does not escalate too much as the years pass. The pricing on keeping a domain private should also be born in mind. Name cheap provides this free with transfers or purchased with a new domain and at a reasonable price when renewed. Their privacy service is called WhoGuard.
For a good one off deal GoDaddy is worth a look for both a new purchase or a deal in transferring a domain. The deal becomes less favourable when it comes to renewal but not as excessive as some registrars.
Note: The links provide here are affiliate links and help fund the site if once you have checked out the deals find it worthwhile to purchase.