Simple 300-400VDC/100A power supply for EV charging from 400V 3-phase/32A
Power factor correction is the hidden biggy with any powerful 3 phase AC to DC converter. It’s simply not the case that the rather elegant 3 phase rectifier circuit can be piggy backed with a boosting power factor correction circuit. This will not work. This is because the elegant 3 phase rectifier (used for many tens of years if not a hundred years) just doesn’t allow it to happen. See this for an explanation. 

This won't work: -



Because the current in each phase falls to zero while its respective voltage is still very high: -



The only viable alternative is to regard the 3 phases as 3 individual supplies and have power factor correction replicated for each. Then there is the requirement to maintain load balance so, there has to be significant cross checking between each phase and finally, each of the 3 boost converter's outputs has to be managed so that it can be adequately merged with the other two boosted DC voltages: -



There is a lot of overhead here and I reckon power factor correction alone is about 50% of the BoM cost for the circuits.

I’m not saying that $5k is a justifiable price to pay of course but given the market and trends and hidden complexity of PFC, it doesn't surprise me at all.