Gold for riding training. Travel time is a SIGNIFICANT time sink while using quests to level. You don't want to wait to get it. In fact, while questing, the second you ding 40/60/70/80 you'll want to hearth and train it IMMEDIATELY! 10304 gold is the base cost, which will be modified by reputation. Also, make sure you're in a guild with the 10% riding speed bonus unless you're a class that already gets a mount-speed buff.
Certain locations in World of Warcraft – primarily inns, capital cities, and private instanced zones like your garrison or class order hall – are rest areas. You know you’ve entered one because your character portrait will gain a “Zzz” icon where your level is normally shown, and because you can immediately log out of the game instead of waiting 20 seconds.
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Stormheim is the zone you want to start it. For some reason Legion treasures are not worth any experience like they are in WoD...except in Stormheim. I haven't been able to find out any reason for this, and it's possible it may get fixed/nerfed at a later date. But for the time being you REALLY want to jump from treasure to treasure to augment your quest experience.
One useful data point is the cost of gold from illicit services. The WoW Token has taken a bite from their business, but these people are still out there, botting trade goods and stealing and pillaging accounts despite Blizzard’s continuing attempts to ban cheating accounts and confiscate illegal gold. The going rate for gold from these outfits is about $15 per 100,000 gold.
Edit 20 It’s been a week since this post and a lot of people are still reaching out to me with questions which is fine. I have had a few different people reach out to me to help make YouTube videos based off this guide and more info in BFA which I’m more than willing to do. Whoever gets the videos up first I will link them here for you guys to enjoy.