Actually, Leurocian confirmed fire/blaze cu spawns exactly 1 in 23,301.
There is, however, every chance it may roll as a paragon, at which point you may as much as double that figure.
Believe it or not, spawning one is actually the easiest portion of the equation.
The hard bit is getting it tame while every other tamer present interrupts your tame attempts, (then guild chat/icq all their buddies to come add to the chaos), and before someone else manages to get it tamed. What most often happens is that the fire/blaze cu is eventually killed off in a spiteful tantrum so that no one gets the critter out of the Weald alive.
While I have given them away, I would not consider selling one for anything less than 150m.