We don't know what's inside PS5 cooling-wise yet, but I don't think the BOMs are close. Sony will almost certainly outsell Microsoft again (particularly as Microsoft rally don't seem to care about hardware sales) and if Sony do produce/sell more there footing begins will be a considerable economics of scale advantage.
I'm interested in what bulk buying advantage you think Microsoft have? Sony produce not only more consoles with optical drives in, but make Blu-ray and AV systems with those in as well. Their phones, TVs, high-end cameras and medical devices all pack solid-state memory and conventional memory. Sony are hundreds of millions of hardware units with these in annually so they probably have a lot of price-negotiating leverage.
Fundamentally, PS5's APU is smaller, so every wafer is proving more viable APUs which is a considerable saving. PS5's SSD is smaller and faster, which probably cancel each other out but this is the kind of thing that will cost reduce quickly as faster solid state becomes the norm.
In terms of the the economics of the launch, it goes beyond the BOM into user investment metrics. Sony are pushing the need for the new DualSense controllers, which are huge profit divers. Idiots like me will be picking up 2 extra controllers which basically amortising any PS5 cost subsidiation Sony would eat anyway. Plenty of folks will pick up at least one extra controller. Sure, Sony eat the profit 'loss' but now I'm a PS5 sure for six to seven years.
It's never been so true that Sony and Microsoft are taking different paths.