No question the Mondos have been the "best buys" in my day (size 2009). The original MP market has been relatively stable comparatively.
I got Ansin's POTO for $150 and resold it 6 pos. later for $600. But it shortly thereafter zoomed to $2000.
*****
When I had to unload my Round 1 collection on EMP in 2015, I made a lot of money on a FEW posters I had found on eBay cheap. Mostly broke even.
But I made roughly $1800 on LAC, $1000 on IMAW, and $1000 on the YOLTs combined. The rest of my best buys I only made $100-$300, ex. Animal House.