Since their value increases for each enemy permanent they take off of the battlefield, you're effectively tripling the benefit gained from casting one
Seeing as most high-level mons were doing heaps of damage at a time, playing a Mr
Can Players Untap Only One Land Per Turn
This strategy was not only the most popular deck and highest achieving, taking four out of five spots in the top 5, it was also shockingly consistent, with a win rate of 56.8%
T: Add one mana of any color to your mana pool
The early prices bear this out, as people move sooner than they should, but its entirely possible that one or two of these dragons follows the Old Gnawbone path and just never gets cheap: Gnawbone is one end of the potential spectrum here, but the other end is this: My inclination is that while the Ancient Dragons are expensive mana-wise, they will generally be worth it on the board