I believe evolution times and food costs are the most important factors in buying a gold dragon. Even at the 800 gold/dragon price, if you get it up to a 10 it'll pay for itself in less than half a year. I looked in the dragon story wiki to get food amounts and evolving times, so hopefully they're correct. If so, here's the breakdown of each gold dragon:
Goldwing:
Food: 125+250+625+1500+3750+6250+15000+35000+62500= 125,000
Evolving Time: 27 hours
Sterling Silver:
Food: 100+200+500+1200+3000+5000+12000+28000+50000= 100,000
Evolving Time: 8 hours
Goldleaf:
Food: 125+250+625+1500+3750+6250+15000+35000+62500= 125,000
Evolving Time: 22 hours
Treasure:
Food: 125+250+625+1500+3750+6250+15000+35000+62500= 125,000
Evolving Time: 19 hours
Rose Gold:
Food: 125+250+625+1500+3750+6250+15000+35000+62500= 125,000
Evolving Time: 21 hours
Gilded:
Food: 125+250+625+1500+3750+6250+15000+35000+62500= 125,000
Evolving Time: 30 hours
Forbidden:
Food: 125+250+625+1500+3750+6250+15000+35000+62500 = 125,000
Evolving Time: 6 hours
White Gold:
Food: 125+250+625+1500+3750+6250+15000+35000+62500= 125,000
Evolving Time: 21 hours
Midas:
Food: 125+250+625+1500+3750+6250+15000+35000+62500= 125,000
Evolving Time: 36 hours
Stainglass:
Food: 125+250+625+1500+3750+6250+15000+35000+62500= 125,000
Evolving Time: 26 hours
I'd say that Sterling Silver is the BEST buy, and Forbidden the next best. Now, for those who want to buy one, I wish that we all have a gold sale before/on Monday!