by definition, an item can only sell at a price that both the buyer and seller agree upon. If you were willing to pay that price, then clearly it is worth that much to you and you did not get ripped off. as a matter of fact, there is no such thing as getting ripped off unless you thought you were getting one thing but ended up with something else of lesser value.
Especially in this hobby, you will see lots of corals and stuff for sale from hobbyists. there is no way to know exactly what something is worth as you usually cant just price match it as you would with electronics. it just depends on how much the item is worth to the buyer and to the seller. For example, i may have a coral that i love and i would not sell for less than $100. someone else may not care that much for that particular coral and not pay it, so nobody gets ripped off. if someone finds the coral to be so beautiful that he wants it ever for $100, then he did not get ripped off either, because to HIM it was worth it.
Now, there is always the opportunity cost involved, which may be what you are thinking about here (and what others have mentioned above). The opportunity cost, is the cost of an alternative. in other words, how much would it cost for a similar product elsewhere, or how much you can get elsewhere with that same money. That just comes down to doing a bit of research before buying something, just checking what others (hobbyists or retailers) would sell such a set-up for would probably be your best measure to compare and see if you are getting a steal, paying the average price, or paying a premium.
There was some discussion in earlier threads about some high priced corals that got sold on Reef Central. but when it comes down to it, there were people willing to pay those prices, so the seller did not need to lower his price.
Hope you enjoyed your little shopping lesson