I like the rule mentioned above. My max is $25, but i did win a bounce so i have one LOL.
That said, i do not know enough to help your actual question to be answered. Good luck and only pay what you are comfortable with.
Side story, i once bought two bargain mushrooms for 10$. For two years i traded in rocks for over $80 that were covered in them almost quarterly. The best was when i got trade in credit for $160. Now i have three mushrooms (ok 4, but when i can get that thing loose he is out) and they all grow slow so i love them. Mushrooms are pretty and if i ever get to set up another tank, it will probably be a shroom tank, but until then; i want other coral that is easier to control.