The most in demand would be Flying/Beast. Probably Critter/Mech next. But in 5.4 you can trade in 3 of the same type for a generic BoA one. So I think stone demand will go down. But you never know??
Not sure of price. For the good ones maybe 3k?? Maybe someone with more experience could say for sure. I haven't had any success trading stones for quite some time. But the only pets I need now are really expensive.
On my realm, the BoE stones are 3.5 to 5K. So, with the 5.4 changes on the horizon, figure a third of that. In the meantime it depends on the family. There are so few Undead uncommon-only pets that people were stoning Ghostly Skulls and selling them on the AH for 100 gold. On the other hand, a Beast or Flying stone may as well be a BoE stone- no one has enough of those.