An AX-12 is around 45€ and an MX-28 225€, that’s a factor of 5.
The differences between AX-12 and MX28 are numerous and very significant when you are interested in making more than just something that more or less moves in the right direct: much better position sensor (magnetic encoder, more precise and does not wear), much better motor (Maxxon ReMax coreless motor, much bettera and with less initertia than regular brushed motor of the same size, meaning better reaction of the servo), better gears, better control algorithm (full PID instead of the limited P controller of the AX), more robust case. Back-drivability is better, and I think that’s because the motor driver used is different, and does not actively break when the servo is powered (in this case, breaking is made by having both terminals of the motor electrically connected together, which makes the servo resist movement when the rotor is driven), though I’m not sure about that one.
And each of these upgrade cost money, though some might be only side-effects others. All in all, the MX are much much better servos all around.
Think about it like a car : the cheapest one you can buy will get you around, but if you want better safety, better equipments, better design ,etc, you have to pay more. You don’t think about Ferrari cars the same way you would a cheap small car, they satisfy different needs (one is there to have fun, show off and get pleasure for its ownership and use, the other is here to just do the job without costing too much).
I’m not saying that the servos of Poppy are not expensive, because they sure are, yet they are very far from the top of the line, and are actually extremely good value for the price.
As said before, nothing stops you from using cheaper alternatives if they fit your needs, I personally use AX-12 and XL-320 Dynamixels for robots and I’m happy with them, and that’s possible because they are good enough for what I want to do. Poppy has higher ambitions and more specific needs, and that requires good backdrivability and reliability.
Care to expend on that? What robot are you talking about?