I'm assuming it will be nearer the cost of the griffon in the SoM book, somewhere in the region of 150, that would be in line with the O&G wyvern which is really the only comparable monster from an 8E book
In any case I don't see much point in buying a combat monster as a mount for a character you desperately want to keep out of combat (of course if the VHS is still in the magic item list things will be different), I've seen the high elf dragon Mage fielded a few times and it has the me problem, no one really seems to know what to do with it




