The million dollar question 
I purchased the spindle around this time last year for $1400USD (no VFD), however this didn’t include the lovely import tariffs, which I think was ~25% at that time, not sure what it would be as of today. This was for 2.2kW 30K RPM ISO20 /w one included ISO20-ER16 toolholder valued around $100.
As for the electronics and pneumatics, no idea I’m sure I’m upwards of around $1.5k. Some of that is from learning and figuring out things as I went - mistakes made, purchasing some of the wrong components. Also went with more industrial components as I felt this would be a long-term invest (and could carry over when I replace the SO3 platform base).
So you got to out weigh upfront cost vs return, how much time is saved with automated tool change vs just buying another SO3, and plenty of other variables to consider. This not being a [full-time] business but more of hobby for me that justification is even harder but basically the money I do make from my SO3 I just turn round and throw back into the machine(s) and has covered a decent portion of it.
I should also have a disclaimer that my SO3 is highly modded (one could argue it isn’t a SO3 anymore and I would agree 
). I have stronger closed loop steppers and linear rails (instead of v-wheels) to support the additional weight as a 2.2kW ATC is heavier than the already heavy typical 2.2kW spindle.