GoDaddy is the cheapest. They always have coupon codes to make .com $7.50 or less. Sometimes as low as $1.
You can try Czech company Subreg, they have sale on com and net. https://subreg.cz/en It's english site. Right now they offer dot com $6 and dot net for $5. I have 6 domains with them. They are good. (I'm giving the real link to real site, not like everyone else apperently...)
The cheapest for the first year is probably going to be GoDaddy coupled with a coupon(google "godaddy coupon" or similar). Other than that I use NameCheap.
GoDaddy, using one of the many coupons that are out there. The other registars can hardly beat 1 or 2 dollar .com-s that are offered there.
Godaddy. You can register a domain for $1,17 (0,99 + reg. fee). Select your domain and search "godaddy 0,99 coupon code" on Google.
Hostingbook.org offers great prices for all .com, .net, .org, .cc including all AtoZ country domain names. Hope it helps you.