Poe Ghostal wrote a lengthy article using quotes from several recent Mattel Q&A’s about the expected price hike for next year. I’m not going to rehash everything he said because I think he did a better job than I could. There were two choice quotes, though, he used made by Scott “ToyGuru” Neitlich, manager of the MOTU brand.
From the Ask Matty forum on MattyCollector.com:

The Horsemen tend to “overdeco” figures. Every figure in 2012 is delivering $22.00 worth of paint apps. In 2013 we are dramatically raising prices so we can afford more paint and accessories. If we wanted to deliver all the figures exactly as the 4HM painted the protos we would need to charge more and we already sold in the sub for 2012, so that is not an option. Legally we can’t raise prices on the sub after it is sold in. This is why we will start 2013 at higher prices.

The most frank and honest quote I have ever heard from a Mattel rep, again from the Ask Matty forum:

We will keep the line going as long as their are customers buying. But, yes, as I stated on Roast Gooble dinner, one day these figures will cost more to make then fans are willing to pay. Hopefully that day will be far in the future.

Again, I recommend fans of the MOTU Classics line to read Poe’s article, which can be found here.