I feel your win, when I first got back into Lego, I wanted all the old star wars sets, but they seemed to have tripled in price.
As much as it sucks for us buyers, I understand the huge inflation you see on sealed sets, especially a 20 year old $100 set.
In terms of supply and demand there are barely any bsb for sale, so of course people are willing to pay more, also don't forget the 20 year job the seller had. They had to move this sealed box everytime they moved, they had to find a safe place for it, and essentially care for it for 2 decades. A lot of members on this forum aren't even 20 years old. So when a bsb comes up for sale for $500, or more, remember someone bought it brand new, resisted opening it and stored It safely for close to a quarter of a century. The price gets a little easier to swollow when you look at It like that. At least to me.
Cheers.