The phrase "to cost a penny" means something is very cheap or inexpensive.
Sentences with «to cost a penny»
It doesn't cost you a penny more, but it helps me out. (humblemechanic.com)
Remember that if you purchase something from an affiliate link it won't cost you a penny more, but it will help to support this blog. (naturopathicpediatrics.com)
They just made the product appear a dollar cheaper and it only cost them a penny. (moneysense.ca)