I wonder what ROI is over the years on a card like a Black Lotus?
So 300,000%+ ROI
But for the most abundant printing, the buy list prices here are reasonably representative (for a one is very good shape - most will sell for less due to condition): https://www.mtgprice.com/sets/Unlimited/Black_Lotus
Honestly, you'd be better off looking at the reserved list as a whole if you want to get a sense of more general returns. It's wild how much things spiked a few years back. I play legacy in paper regularly and have a large collection I actually use, but it's getting to the point where even I feel like I should sell out.
Looks like it floats around a 20% to 35% APR depending on the years.