I think the cheapest you'll find is maybe $6.75 per name but you'll have to look around or just ask your current registrar for a discount.
Also, you may get what you pay for. Cheap registrations probably means no or bad customer service, bad user interface (making changes difficult) or hard to get auth codes so it is hard to transfer out.
Go with known registrar who you have dealt with and like the user interface etc. Transfer one domain to five or six different registrars then make changes etc and see which one you like the best.
Price should only be one consideration.
I am a wild west reseller (godaddy reseller) and like the interface and get a refund on every name I buy. I think my net is about $7 on .com names which is about as low as you are going to get.
It costs $99 per year to be a godaddy reseller.