I bet at least like 9/10s of the people who read this board own a Big Muff pedal.

yeah, used prices for sovtek and ehx stuff seems to have gone crazy recently - old electric mistress pedals are going for like £250-£300 too

Old Big Muff prices can get pretty high. Variations in components between different production batches led to different tones being available, some more well-regarded or highly sought-after because of whatever famous guitarist used that particular model. Add to that relative scarcity because the company went out of business a few times. Or people would buy them and stomp them to death and discard them. There is an exhaustive history that tbh just makes my head spin. The section on the Sovtek era starts here: http://www.kitrae.net/music/big_muff_history2.html

There are many modern companies that have been making clones of and improvements on many of these old muffs for years. In fact, EHX is notably late to the parade by rereleasing their Green Russian just last year. But I guess the old ones still retain their collector's value.