It costs the registrar more than $5 to get a name with the registry+icann charges.
Any real registrar who gives you 12 months for less than $5 is
a) laundering money
b) implementing a loss leader strategy to gain market share or going to hook you for something else (hosting, renewal fee, domain lock etc)
d) going to take your money and run
e) going to be out of business fairly soon
If you go with any of the above then don't be surprised if something happens that will end up costing you more than $5 in the future. You coudl also try signing up for a reseller/affiliate account with some registrars and if you're good at pushing sales you effectively get your names free.