I wish it told you which you already have when you got to breed so you don't waste your time breeding for ones you already have unless you want to.
For that situation, do you not look at the Dragonarium first? I wish the Dragonarium would 'highlight' the breeds you have instead of 'shading' them, that still confuses me.
I make it a point to breed two of each dragon, the second of which ends up as a dragon egg pedestal. This way when the breed cave completes and the egg arrives at the hatchery I already know if its a new breed for me or if I need to make another go at it.
After seeing the prices on this game I was sceptical and then after this special edition dragon being released at an insane price I deleted the game pretty much instantly after seeing it. These devs are deluded as must anyone be to pay the prices they charge.
I suspect your referring to either the rainbow or the hyroglyph dragon. IF you go in the dragonarium and find either you'll see that both are perfectly obtainable via the breeding caves. Rainbow wants four elements present in the breed (2 on each dragon or 3 and 1 etc). And the glyph dragon comes from earth and fire breeding. Dragonvale's rarer breeds have always been over the time in pricing in both games, but the dragons can always be breed for free.
Ah I never played the last one so I didn't notice that in this one. I was doing fairly well but I still don't think I'll bother with it again it wasn't much fun for me sadly which is probably why I didn't stick to the other one more than 5 minutes.