There's a sign outside the entrance that reads, "It is believed that if you spend the money that has been washed in the spring's water, it will increase many times and come back to you. The spring is one of Kamakura's five famous water." The money has to dry naturally though, and has to be spent in order for it to double...at least that's the belief.
If you've got a Japanese Shrine Book make sure to bring it with you. We both got our books stamped. We visited another shrine that day and I took video of the man writing in my book (at the second shrine). I love watching it because it seems so simple and effortless, but when it's done it looks beautiful! I've included a video of it below. Those who subscribe via email may need to go to the main website (http://www.passthechopsticks.blogspot.com/) in order to view it.