How much is 1975 penny with a stamp above the date that look like an islands?

If you're referring to a U.S. penny, it's almost certainly a privately-made novelty item.

There are many companies that use metal punches to add pictures or Words to ordinary coins - state outlines, birthdates, etc - and then sell them as collectibles or promotional items.

Usually the coins come in sets and there is a niche market for complete sets. Individual ones that have been removed from their sets are considered to the altered coins and have no extra value to collectors, however.