Friend, please clarify: is "the beach" a euphemism (a joke) or do you have a coral store called The Beach in your area? I think you're confusing all of us here lol!
You're original wording did kind of make it seem like your girlfriend was out scouring the local beach area looking for just the right coral for your tank, and the only thing I could think of why she may need $20 for is lunch on the boardwalk :)
I do think the other replies have really said it well, there's lots of easy, inexpensive corals for your tank, but don't take it from us, do your research so you know for sure what you're getting and how to place it and care for it. There's lot's and lot's of info out there, countless "Top ten beginner coral" videos and posts here on R2R that should help guide you, just take a look for them.
And to me, there's nothing wrong with doubling up on something you already have, especially if it's doing well in your tank. For example, you have a gold hammer, maybe get a green one, or a frogspawn that should also grow well in similar conditions? You have Zoa's, they come in a near unlimited amount of colors. Nothing wrong with having two or three of something that's doing well for you, especially if you have some different colors.
As always, best of luck with whatever you guys get!
