Prices are always changing and there are always special offers / discounts – so we couldn’t give you a definite answer as to “the cheapest registrar”. However, there are a number domain name registrars who provide regular cheap registration for .com domain names you’ll find a few of the popular (and recommended) ones listed below.
At the time of writing this answer, those are the cheapest places to register a .com domain name. Name.com is probably cheapest at present.
Popularity: 7% [?]