You still have to upgrade your riding skill from a riding trainer in order to upgrade in mounts (regular ground mount, epic ground mount, regular flier, epic flier). The gold needed to upgrade riding is MUCH more than it is to buy the mount itself. I'm only level 40, but when I bought my Kodo, I paid 35g for the riding skill, and 10g for the mount. Same price for the mount, but at level 60 the riding skill costs like... 550g approx., don't quote me on that, because I don't know for sure. I forget how much regular flying costs, but the epic flying is somewhere in the neighborhood of 5,000g. Have fun farming for it!