Median price for a beta Black Lotus in 1994 was ~$15 via Scry Magazine. A lightly played beta Black Lotus currently goes for ~$45,000. Alpha ranges from 150k-1m+ based on grading.
Alpha and beta printings are more rare and the exact price is hard to get a handle on.
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.
