I measured that some years ago. It was ~60W or so pulled through the slot for a card with two extra cables running to it. Specifically I measured the current down the wires and subtraction gave me that as a reasonable estimate for the slot. Won't be perfect but pretty sure it's non-zero through the slot.

Right! e.g. for RTX 4080, TGP is 320W. Discrete cards definitely have a bigger power envelope.

75 watts

You can use GPU-Z to show the pcie slot power draw vs the PSU cable power draw