RSMyth here...I was think about this idea for a very long time and finally decided to make it. This is what I came up with, it only takes 4 coins, has a reject button, and a way to see how many coins is in the machine. Expect more soon because starting a new minecraft youtube channel and will make crazy contraptions! For all that's redstone :) Peace.

View the video of the map here (recommended):
http://www.youtube.com/watch?v=TUPJDXFoODA&feature=youtu.be