1. A flippant response given to a monetary question to which the person answering does not know or care. In this context it is typically used to be insulting towards the considered value of an item.
2. The sum of money offered as a dare to encourage somebody to do something they wouldn't normally.
In either context the term generally refers to the sum of money in the form of a single 50p coin which, in the British Stirling currency system, is a heptagonal coin with curved sides.
Q. How much should I sell my old laptop for?
I'll give you fiddy me if you help me.