With most people having a level 80 including myself I am now 'working' on characters number 2 and 3 to level up to 80 as well as working on getting the professions back to a useful level.
Unfortunately quest rewards from levelling my level 72s are just about enough to cover skill training and don't really contribute to levelling my professions much.
In other words: Professions are bloody expensive.
And even worse: I feel compelled to have each profession at at least a decent level to reduce my dependency on trading with others.
And while my AH business ventures are doing better than ever since I actually do invest a little time in them these days I regularly think about the fact that on my level 80 warlock I still have a large chunk of zul'drak and all of sholazar basin, icecrown and storms peaks left to do.
So how much gold could I potentally earn at level 80 from doing quests?
Before we dive into the calculations we need to be aware of a few facts:
1. Gray quests do not give any gold beyond that what is advertised in the quest text. So if a gray quest normally gives 45s and 600xp it'll give you 45s at level 80.
2. We know from wowwiki and various other sources as well as personal experience that 1xp is worth 6c at level 80. so for every 1k xp you can expect 1k * 0.0006 = 60s.
3. Most northrend quests give between 20k and 25k xp at level 80 depending on your zone meaning you can expect between 12g and 15g per quest excluding any gold you may be able to churn out by selling item quest rewards.
4. And this is an important one. If a quest says you normally get 5g and 25k xp you will only get the XP->gold conversion and not the base quest reward at level 80. So you will get 15g from this quest instead of 20g which is different from how it worked in TBC (*citation needed).
Next up we need to figure out how many quests we have per zone. On your way to 80 you either finished off Borean Tundra, Howling Fjord and the Dragonblight or not, either way you will notice that most of the quests in these 3 zones now are 'gray' to you and thus won't give you any xp->gold reward.
Grizzly hills is a bit of a toss up really since it's going to be right on the line of gray so it'll have a fair few quests that still give reward but an almost equal amount of quests that are gray already.
So the relevant zones remaining are:
Ignoring dungeon, dailies, raid and pvp quests so only looking at the group and normal quests wowhead will happily tell us the amount of quests per zone:
Icecrown - 250 quests -> 140 for achievement
Storms Peak - 159 quests -> 100 for achievement
Sholazar basin - 101 quests -> 75 for achievement
Zul'drak - 130 quests -> 100 for achievement
The reason I listed the amount needed for the achievement is quite simply because some quests are part of elongated quest chains and others can be damn near impossible to find. The achievements are however fairly easily completed and can be a good guideline to what can be achievable without investing hours upon hours in researching various questlines and travelling all over the map.
There's no point in going into detail on how much xp each quest gives and what the corresponding rewards would be since we're only looking for a rough estimate of what the total gold reward per zone would be.
As a result we're going to go with the value of 14g for each quest completed in northrend. This of course excludes reward items which may be vendored or not for additional rewards. Some of the quests are probably simple crumb-trail quests to send you from one quest hub to another but for the purpose of simplicity we will simply assume that they too earn you 14g.
What can I earn?
The low end represents the approximate gold gains for doing only the amount of quests needed for the achievement. The high number assumes you complete all normal and group quests in the zone (which is somewhat unlikely).
Icecrown: 1960g to 3500g
Storms Peak: 1400g to 2226g
Sholazar basin: 1050g to 1414g
Zul'drak - 1400g to 1820g
For a grand total of: 5810g to 8960g
This is excluding drops, item rewards and other rewards that may be turned into gold and represents and estimate. Taking extra item rewards and drops into account you can probably strap another 50% on those numbers meaning you're looking at 8k to 13k gold for those remaining 4 zones.
All in all those aren't bad numbers, especially not if you're interested in completing quests merely for the lore aspect beyond 80.
If nothing else it's good to know that you can easily pick up an epic flyer by doing nothing more than what you've been doing all the way till 80 (quest, quest, quest...)