I think the original price of 15 bucks was a really good spot for these. It's more expensive than similar-size lines like your average Playmates current-cartoon Ninja Turtles, but the MOTUO bodies had more articulation (i.e. more parts in the tools and more assembly at the factory), more complex engineering (because of all the interchangeability), and were generally chonkier. So, you know, a few bucks more than a Playmates Ninja Turtle with 7 POA that don't even work properly? I can do that.
Realistically, I think Mattel just got greedy. With other price increases I can break that down to other factors - some of which are just 'greed, but with a twist.' But in this case I really do think it was JUST pure greed. Mattel saw the line go gangbusters almost immediately, and within the first year they ratcheted up the Canadian price first, and then increased both the US and Canadian price again after that in very short order. Then they powered through retail releases, jumped to mostly just selling online, and pushed the price up again.
This was nothing more, in my opinion, than Mattel saying 'I think we can make more money on these customers - they seem pretty dedicated, so let's test that.' It's pretty fucking gross, actually.