Are you looking at full working systems or just cases?
It does seem like slot 1-type stuff on eBay is priced quite high, but I think the big issue is that slot 1 is increasingly rare. Most slot 1 stuff got thrown out 15+ years ago...
Also worth noting - the slot 1 era happens to be a time when the big direct OEMs (Dell, Gateway, etc) were selling very modular systems. So you can make an enthusiast retro system out of a Dell Dimension R or T or the Gateway equivalent in a way that... you couldn't... with some integrated graphics/onboard sound/mATX POS from 2002-3 or an "IBM" (nee Acer) Aptiva from 1998. And with enthusiast motherboards for that era being rare too, a full ATX Dell/Gateway/etc system is not a bad alternative option.