"All other things being equal" is tough to find, but there is one pair of car sets that allows a direct comparison. Looking at photos side by side comparing the eight wheel 653, 654, 656, 656 and 657 hopper, tank, box, cattle and caboose with the 803, 804, 805, 806 and 807, it looks like the same set of bodies on both series.
Here are ranges of recent sales on eBay
653 10 - 20 803 15 - 30
654 10 - 15 804 15 - 25
655 15 - 20 805 20 - 30
657 10 - 20 807 5 - 10
Draw your own conclusions from this. Thinking about sales that I've made, I'd say the 4 wheel is worth maybe a third more than 8 wheel.
