Hey folks, discovered this at the Glitch page of the wiki
Some dragons sell for different amounts from the Hatchery as opposed to from a habitat. Most sell for less, but Iron currently sells for more. You get 2 million if you sell an Iron directly from the hatchery, but 1.5 million from the habitat. At both points you get a message saying "Do you want to sell for 1 500 000" but you get a different amount if selling from the hatchery or the habitat.
So, breed Irons for cash and sell direct from the hatchery is the advice at this point. They are the best deal going whilst this glitch is in place.
Mags only sell for 1 250 000 from the hatchery (1 500 000 from a habitat), Cactus sell for 10 000 from the hatchery (100 000 from a habitat).
The complete list is at
Glitches - DragonVale Wiki