I doubt the number of bots in the economy has changed. Why? I have a miner. I see bots every day in Uldum. I report them every time I see them. Months and months of doing this (heck, the entire expansion), and some of the same names are still showing up in Uldum.
I'm not talking about legit players.
I'm talking about the player you follow, attack just as he's trying to mine, watch him stand there jumping, trying to mount and fly off, then once dead, takes rez sickness, goes right back to the same route, and next time you find him, attack again. He just stands there jumping, attempting to mount and fly away.
No, Blizzard hasn't really done a whole lot about the botting problem. This does not mean that the supply has remained perfectly constant.
No, supply does fluctuate, but not because of bots waxing and waning. Supply fluctuates because non-bot players are not always consistent about when they go out to mine. Maybe, on my miner, I feel like doing a bunch of dungeons one day, and I spend my time in queue mining in Uldum, and then offload it all on the AH. Or maybe I don't, etc.
But there's not really a pattern to this non-consistent supply. And it doesn't really go far to explain the price curve.
Less people gearing up has an effect on velocity gems and other enhancements will sell at (number of sales per day) but it has little effect on what someone is willing to pay.
While it is true that fewer people gearing up has little effect on what someone is willing to pay for gear enhancements (gems, enchants, etc), the statement is a bit misleading.
I am a raider. Part of raiding means that my gear will be properly gemmed, enchanted, and reforged. This means that what I am willing to pay for my gems is essentially whatever the market price for the gems are at the time I need the gems. If I need to fill a socket with a red gem, I'm going to buy that inferno ruby, whether it's 70g or whether it's 200g or whether it's 400g. Now, at a certain point, I might compare the price of Queen's Garnet to Inferno Ruby and decide to jump up to the Queen's Garnet, but there's no price you can set red gems at that will completely prevent me from purchasing a red gem when I need a red gem.
And I'm not alone.
This is not to say that increasing price has no effect on the velocity of sales. That'd also be incorrect. The market is not composed 100% of raiders. And even among the raiders, not 100% subscribe to my idea that "These sockets must be gemmed!" But enough do.
The point I've made here is that, no matter the price, there will ALWAYS be a demand for red gems. (I mean, I suppose at a certain point, if purple and yellow gem prices remained constant, people may just opt for these rather than pay 5,000 times more for the red gem.)
So far, I think I've pretty well established that SUPPLY has NOT changed, and while PRICE may take us to a different part on the demand curve, we're still on the demand curve, no matter what we set the price at. There will always be a demand.
So now we must look at DEMAND. This is the only part of the equation that can explain a price leap up to 400g at the beginning of the expansion and a drop to 70g now (on my realm, it's still 90-120g, but still a significant drop).
Okay, so we've got a few different level of players.
We've got heroic raiders who were 7/7H in Firelands for a long time, all 391 gear. They full cleared normal DS in the first week and have been 8/8H in DS for a long time now. In the 5 months since DS released, they've gathered pretty much all the loot there is for them to gather.
We've got the Normal Raiders (like me). When 4.3 dropped, they perhaps had some upgrades in the HoT dungeons, but primarily, their upgrades are coming from valor, LFR, and normal raids. By this point, if their raid progression has slowed down, they've probably got just about all the gear they're going to get. And even if they do get more, it'll certainly come more slowly now.
We've got non-raiding PVEers. Before 4.3, these people just ran dungeons. So with 4.3, these people have a new set of upgraded gear between HoT dungeons, valor, and LFR.
We also have PVPers. No matter what level of PVPing you do, there was a new set of honor and conquest gear with 4.3.
And let's also not forget, we've been getting a massive number of Scroll of Resurrection players. These players can fit into any of the categories, plus they're quickly adding a character via a free level 80.