About $100. Way cheaper than a "real" BMS. And arguably better.
The most expensive bits are the DC-DC converters @ $18/each X 3 from Digikey. Nanos are $7.75/ea here:
Mini USB Nano V3 0 ATMEGA328P 5V Micro Controller Board FOR Arduino Compatible | eBay
EDIT: I just bought two of these Nanos! Ha ha.