It looks like the shrimp are $10 on live aquaria, my LFS
sells them for around $20. The macro algae isn't that much, the interesting looking stuff is a bit more and from what i have read if not properly maintained can go sexual and crash the tank. I'm sure a LFS
would order some macro in for you but I haven't ever seen any at mine.
These are the links to live aquaria.
If you do decide to do this the most important thing I could think of is to make sure the salinity of you new water for water changes matches the tank exactly. On such a small tank it could change the salinity pretty rapidly which can kill inverts. The other thing would be to research the proper maintenance for macros, I don't have any experience which them.