I estimated the difference in BoM between the two consoles a while ago, and my conclusion was same as yours it should be around $50-60 more costly to built. But, the majority of savings would be from the APU, rather than rest of the machine. I'm sure you remember about AquarisZi? That guy was first to tell us about PS5 SoC size at ~300mm2. His guesstimate along with Github had so many peddling the $399 tag and the power differential in next-gen OTs. As time has passed on, I'm leaning towards his suggestion being right. 40CUs are tiny, and their I/O customization won't take up much space on the die. It could very well be the smallest SoC every built for a PlayStation, and compared to Series X save them most on cost.
RAM is easy to guess, as MS are simply buying 2 more modules. The same goes for PCB cost, due to bus and two separate motherboards.
SSD, I think Sony are spending more, simply because flash is expensive, and they are buying 12 of those modules albeit small, compared to 4 for Series X. Same goes for some other peripherals, such as the controller and the more fanciful box design.
Cooling is an interesting one, because from the patent we saw, the heatpipes will go through the board, them using liquid metal compound is another cheaper yet effective method of cooling. But, it's tough to estimate if Sony are saving any money here until we see the tear down.
Overall, as time passes, I'm more convinced about Series X being sold at a premium to PS5. I just haven't got the feeling from MS, that they are desperate for us to buy their box, the interview posted today, is along the same lines. Logically too, why would you spend hundreds of million of dollars in R&D for a new (lower-spec) console, when you can keep the cost down on the more powerful model? Or you have a blank check from MS to eat those losses. It just doesn't jive. Spencers quote on not being beaten on "power and price" will be right, but it won't be true in fashion many were expecting i.e. Series X will be more powerful yet at par/cheaper than PS5. I think he was simply eluding to their next-gen dual SKU strategy. There was a monster energy promo which had Series X + Halo Infinite bundle which came down at $599 in total, if you take the cost of physical copy of HI out. Then it's possible we might be looking at cost of Series X at $549.
I'm gonna stick with my prediction of $549 Series X, $499 PS5, $449 PS5 DE, $349 Series S.