I would guess the Prophet 12 and newer uses modern technology. Prophet 12 does not use DCO's, it's DSP based Virtual-analog oscillators. The Pro-2 is expensive because it uses the DSP Oscillators like the Prophet-12 and it uses two descrete filters, which is more expensive to make. The Prophet '08 and REV2 uses IC's, basically synth on a chip. This makes it cheaper. I would guess the REV2 keybed is more expensive then the oher lot since they've started using Fatar keybeds, which are better in my opinion. DeepMind12 has it's limitations. Since they have different features, they don't compete equally. Personally the REV2 would be a better choice for me over the DeepMind 12 because it has 5 octaves, more voices and more oscillator options.