For Free On The Cheap

It means you bought or traded something and unintentionally received more product(s) than what you paid or traded for. That additional product(s) was therefore “Free” however there was a transaction of money, goods, or services required to receive the free item therefore it was “on the cheap.”

Yo man I bought a 6 pack a socks and there was 7 pairs in there, got that 7th one “for free on the cheap.”