I also broke down and bought the Storage Tower for the goal reward (I just bought a bush and shoved it in there, lol). I also got the Hibernation Cave and the Fountain of Youth even though I'm probably not going to use them (although Blitzen was so cute as a baby I'm kind of tempted to use it on him, lol). And the Light Dragon boost. I'm bad when I have money. :p As you said, at least Diamond fails sell well and I can always profit breed for a while for now.

I'll hold off on the Dark Dragon Boost (the last non-gem building available) until I a) buy a Dark dragon but I won't be doing that until I can b) buy the Seventh Island so that I can c) get them away from the Light Boost.