there are many factors that determine the price of a rare fish. as rare as these are, half of it is determined by the attitude of the person that breeds them. if they are keen to get them out there & make them accessible to everyone then they will sell them at a cheaper rate, if they want to make $ then they will be offered at a higher rate. If more than one person breeds them then there is competition on price providing they don't price fix, then it comes down to how many people are prepared to set up to keep the zebras.
If there were 100 available at $500 each i suspect that they would sell quite slowly as they may not be suitable for some tank setups or current inhabitants, if they were $100 each people would be more likely to set up a tank specifically for them. I would be very interested to see how many people would actually get them if they were $100 or less as from what i understand of their behavior they aren't seen much so the idea of having a tank to enjoy their brilliant colouring may not be what it seems to someone that has not kept them before.