over here in Hong Kong the most expensive fish i've seen for sale with actual price listed was probably a Gold Aro that looked to be around 3'+ in length and came with a price tag of close to if not just over USD $10,000 (that was around 4 years ago)...
Although with that being said, i do like to look at a lot of the stores that specialize in rare or expensive fish and know there are some that are sold for even more than that... but the problem is they will very rarely if ever list the actual price of these fish...
For example i remember when batman rays first became popular out here a few years back, there was a Leopoldi batman ray which was being sold for close to USD 20K at the time, and just last weekend when i was out there i saw a true platinum gar, which was housed with some other rare an expensive fish.. needless to say there were no prices listed on the tanks, although most fish in this particular store tend to range from around 2K USD and up...