So, we all know that prostitution is illegal in real life, but it's a little different in WoW. Hear me out.
On my realm, queues for a heroic instance can reach 45 mins to an hour very easily. Players who are just starting out are going to want to chain-run heroics to get gear/justice points very quickly, in addition to any heroic drops that may happen.
So where does this leave you, oh class that can tank or heal? Well, on the flipside, heroic queues for healers or tanks are much shorter, if not instantaneous. You lucky few are able to carry your friends to victory and shorter queue times. But what if you don't limit your shorter queue privileges to just your friends?
Post in Trade Chat, "WTS Tank/Healer for Heroic Random, 3-5 min queue! 100g!" I guarantee you'll get a reply. When it comes down to the very basis of the economy (especially WoW), you pay for convenience. Sure, you could plant the seeds, water them every day, and eventually harvest the fruit. Or, you could go to the grocery store and buy the fruit from someone who has already grown them.
In short, never run a heroic by yourself. You'll always be able to profit from someone else who wants a short queue, and you'll get the same rewards as if you ran it yourself.
Let's look at this from the other side, though. Let's say you're a DPS class who can't tank or heal (rogue, hunter, mage or warlock), or maybe you don't want to. Don't fret, you won't lose much cash by buying that tank. Completing a random heroic instance will net you a tidy sum of just over 80g. Just for killing the last boss! Never mind the regular loot that drops, the shards, money drops, grey drops, etc. Assuming no deaths in a heroic instance, you'll easily, EASILY make back the 100g you spent on a tank. There's an opportunity cost here of 100g, but the actual cost is just about 0g, if not negative. Think about it this way: If you were driving to the store to buy oranges, and you bought the oranges, you'd be out $4. If you were driving to the store and a man stopped you on the way, said "here, buy me a bag of oranges too" and handed you $10, you would still buy your oranges and make out with $2 in the end.
-If you're a tank or healer, never run a random without selling another slot
-If you're a pure dps or don't want to heal, you could buy the tank anyway and the only downside is you wouldn't MAKE gold, but not "spend" any in the long run.