The price for motherboard is likely to include the cost of obtaining a whole new Xbox, disassemble it without damage, removing the motherboard and laser daughter board to resell at $599.
If you think there are new Xbox motherboards sitting around waiting to be supplied for repair... if I told you the terms ifixit had to follow for consignment of samsung screens? You'd be skeptical too.
At this point, xboxes are more valuable as a whole unit. If you want a mobo, you gonna pay a premium for a company like ifixit to get a whole xbox and part it out for you.
They also gonna benefit from having extra parts on hand from each unit. Things that aren't mated to the mobo, like the psu, disc drive (sans daughter board) and shell parts. Suddenly a $500 xbox is capable of $750+ roi or better.
I could be wrong, but I will share for you the alternative method for parts supply, and it's worse. Samsung consigns entire parts to ifixit, forcing them to do full battery replacements whenever screens are replaced.
This creates additional waste and prevents users from reusing a battery. The consignment also requires customer information to be sent to samsung, enough to identify an individual AND their specific device in a sea of people/devices...