Increased demand, and legal reduction of supply.
Part of the reduction in supply has to do with tighter global arms controls. Gun control is getting bigger and bigger, especially in foreign locations, but it also has increased domestically especially at the import/export level.
The ATF change on barrel importation made a huge change to the import market, and AKs were one the largest imports hit.
AKs were so inexpensive globally that you could travel to many third world nations and pick up one (military full auto models) for a few hundred dollars. However pressure from Europe the UN and the US government to crush these sources and supplies of arms has increased dramatically as well.
So much so that the price has nearly doubled as well.
While you couldn't just import these to the US, they are tied to the parts kit costs, which ties into the total cost of many American assembled AKs you might purchase.
For example lets look up Somalia AK prices:
http://horseedmedia.net/2013/02/12/somalia-illicit-gun-trade-barrels-ahead-in-mogadishu/
The price of guns has more than doubled. Two years ago, a second-hand AK-47 used to cost $350. Now it costs $750. The price of bullets has also shot up. A single AK-47 bullet used to retail at $0.50, but now costs $1.
Izhmash was also producing more AKs than the Russians needed, and they would purchase them and send them directly to storage.
They have millions more AKs than they use if I recall. It was only a matter of time before that stopped. How long would the government keep buying weapons it had that little need for?
Last year they declared bankruptcy, though they are still around producing.
Global gun control and demand have combined to increase costs.
Domestic import restrictions on barrels per the ATF plays a big part in the cost increase at home.
What remains of those with barrels since the 2005 restriction imposed by the ATF has been drying up in the panics since then.
The cost of a brand new barrel made in the USA is more than what an entire parts kit with included barrel cost just a few years ago when there was still a lot of them. So the 2005 restriction has finaly been in place long enough to be felt in the AK market.
Previously AK receivers could be bent into shape, finished to look how the consumer market expects, a parts kit used to complete a rifle, and then be put on the market costing something like $150 in parts. Then the cost of labor, maintaning a business, and profit bringing them up around $250-300 retail.
Now just a cheap US made AK barrel is over $100, a cost addition that didn't use to exist, and at the lower end is not as good for abuse as the old milsurp chrome lined barrels. Chrome lined accurate barrels of a decent alloy up over $150. So just the barrel costs more than what it used to cost in parts to make the entire gun.
Add in global gun control pressures reducing supply resulting in what used to be excess being diverted elsewhere with increased demand.
And increased cost to do business.
Toss in some panics and increased US demand.
With that source increasing in price, and less price difference with direct imports or conversions, the demand on other sources like direct imports also increases. So they go up in price across the board.