I started with the 7 gold boost because I had both nests, evolution temple, and everything in arctic isles ready to collect. Now I've done all that, fed a bunch of things (finally got black knight to level 15) and I'm done. I don't care much for the final milestone dragon and have most of the rest, so I don't need to spend more gold.
I do like the crescent dragon but I don't want to spend a ton of gold to fight for it. One other person in my group also started boosting, but they chose the more expensive boost and have five gold dragons to my two.