> What's the difference in cost between a cable that can do the bare minimum, and a cable that can do everything, assuming they're the same length?

A USB-C cable that can do the bare minimum needs five wires (one twisted pair for data, power, ground, and the configuration channel), and they can all be thin. A cable that can do everything adds to that four more differential pairs (which also have more stringent signal integrity requirements than the lone USB 2.0 pair), one extra pair for sideband use, and the power and ground wires must be thicker to handle the higher current. So it's tripled the number of wires, made some of them thicker, and the cable has to meet a more strict signal integrity. No wonder it's more expensive.

That depends on the chosen wire gauge and length of the cable, but even the most cheaply made cable would be significantly more expensive if it supported all capabilities. You can't go lower than the cost of materials and cables necessarily use a lot of copper and plastic. Adding support for more capabilities requires adding more wires inside. And the more wires you have inside the cable, the more you have to worry about cross-talk, so you need even more material for shielding and you need to twist the wires tighter (and a tighter twist shortens the wires so you then need those to be longer to reach the same distance).

A couple of years ago I couldn't find a cable that did the max speed with max charging for any price. Now looking it seems like you can get a TB4 with 100W 3' for ~30 dollars and 6'-7' for ~60 dollars. Or a 5 pack of standard type c cables for ~10 dollars.