I have had bad experiences with the B250 Mining Expert back in 2017/2018.
I like using a 500-700w ATX power supply for the MOBO + 1 or 2 video cards, then HP server power supplies for the remaining video cards. That is the cheapest combination while still having Gold/Platinum efficiency.
The 850w was the best price performance, not sure now. The asus b250 mining expert used to be 30 usd before this last bullrun 2021, not sure even if the make them anymore, last I heard it was discontinued, that motherboard got as high as 1k n 2021, last I saw, insane people paid that much for it, more money than sense I guess.