If you start with something or get into it early, you have the potential for pretty massive profits.
However, over time, others move into the market and compete which drives prices down.

For six months or so, I was doing Arbitrage on my server.

I made an excel spreadsheet of most items in the game and what they could disenchant into.
If the range for a given item level armor was 80% chance of 2-7 dust, my formula was (2+7)/2 * 0.80 * 0.95. But almost everything had a dust, shard and essence chance on DE.

Then I would scan the AH for a 10 level spread of gear.
Mainly interested in green armors for one scan and green weapons for another.
For a month or so, you could buy blue armors and DE for a profit too.

My auction mule was up to 22,000+ items disenchanted when I got out of that market.

Initially, dust would average 4g each, selling singles.
And I could easily sell an average of 500 dust per day.
It was an awful lot of time wasted for DE's, but that's what DVD's are for.

However, with competition, the price for a dust is now less then the cost to buy the greens.
I cannot make a profit. Too many idiots driving the price down.