Well, I want to retract my statement...
The most expensive fish I've brought were my two mandarin dragonets. They were very cheap to acquire (about $15 each), but expensive to keep alive and thriving. 6 months before I purchased them, I added an additional 50 pounds of rock to my already mature 225g DT giving them more property to graze (approximately $150.00), and spent another $150 on copepods to add to what was already in my tank. About every other month, I spend another $50 on copepods to add to the tank. I'm not happy unless I see the empty shells of a newly molted pod floating around my tank on a regular basis. I usually throw the sponges that they come in in my sump so the multitudes of baby pods can continue to grow (later I use the sponges to seed my QT tanks when I get new fish). 2 years later, I have 2 very fat and happy dragonets that spawn regularly, and my female appears to be pregnant as well at the moment. I'm pretty sure none of the babies will survive, but these two dragonets are definitely thriving.
They eat pellets as well, but I have several pod eating wrasses in my tank, so I want to make sure everyone is well taken car of...
Price of the fish, about $15.00 each. Price of caring for them, probably north of $1,000.00...