I'm currently using an 85 Frost DK, in close to full Conquest gear (from the 4.3 season).
So, lots of stamina... but not necessarily optimal PvE stats.
Still, absolutely no issues killing mobs through level 80 instances with four toons either parked near the zone in or on /follow to me.

I've been transferring 4k, from my banker to my DK.
And then giving 1k to each of the four toons I am boosting.
With the exception of the Paladin/DKs and the Druids (who bought epic flight and faster flight), the characters are fairly close to 1k gold at 80th.
It is close to a break even, for Slow Mount > Fast Mount > Slow Flyer > Azeroth & Cold Flying.

The Epic Flight & 310% Flight will set you back, more than your income from 1st to 80th.
That's also assuming instance boosting, selling BoPs and mailing all BoEs to an auction mule.
Who does not include the BoEs against the cost of Slow/Fast/Slow_Flying/Azeroth/Cold.