[kirrent]

They're large or larger than a double decker bus. They're released well after first stage separation so they need to be as light as possible. Stupidly light. Despite being very large and very light, they need to be very resistant to vibration through hypersonic speeds and go through a huge range of temperatures. They need to separate reliably and equalize pressure during ascent. They look like a mere shell, but they're actually advanced composite structures with a fair amount of tech involved.

[robin_reala]

They also have a built in parachute deployment mechanism which can safely get them down to sea level.

[gpm]

I don't think it's fair to count that as part of the 6M cost that you are "saving", if you just did the typical thing and threw them away you wouldn't need parachutes.