Be sure to include the value of the pony itself, if it's an alt color rehair, I think it should cost more than the base value of the original pony, with flaws considered.
If all trace of the original pony is hidden, then its original value shouldn't factor, obviously.
I would say take a look around, see what other customizers successfully charge for their work (not just what they ask, but what it actually sells for). Try not to get caught up in comparisons, because that's a trap, but do see if you can objectively compare the quality of work, and if you think you're doing a similar quality level, try asking similar prices. (Try to also factor in whether their sale venue is going to charge them fees that they may be factoring in, such as the high fees eBay charges.) You can always start a little on the low end and increase prices as you gain confidence, too. I slowly increased my custom prices as I got better at painting and rehairing and there still seemed to be demand. If it's not selling, try a markdown? Just be patient with it. It's artwork, and that means there is a huge variable in the equation: the whims of the audience.
