BVP-4 Plus:
MSRP was about $750, I have a receipt. Resale sometimes touched this.
2000s resale was about $425 for units in good shape.
Early 2010s resale was about $225, but this was a lull in video gear. Some people picked up cheap TBCs, capture cards, etc, during those days. Those days are long gone. This is when I picked up my own MIB unit for $450, all original documentation, disc, receipt, etc.
Late 2010s saw $425 come back.
Then something odd happened. There was a Reddit-fueled surge for "artistic video" (intentionally damaging video for art), I forget the exact terms they use, or even the subreddit where this all goes on. Many of them are on this site, as members or lurkers/readers. They began to snap up BVP4 units, as they truly are some of the best for this weird neat hobby, as the proc amp has violent illegal value options.
So current price should be in the $450-750 range.
SignVideo/Studio1 proc amp:
These have never really sold below original prices, unless broken.
And
eBay has had quite a few broken units in the past decade, mostly irreparable. Hamfisted moron users (often kids in school/college) turned knobs roughly, and it broke or damaged the internals. So knobs didn't work, or the signal went wonky during turning.
eBay sellers are also idiots, so these got further destroyed by awful packaging for shipping (no padding, incomplete padding, wrong padding).
These were somewhat fragile units, so even well cared for units sometimes had issues.
A flawless unit is fair value of at least $300, the MSRP. (There actually was no "MSRP", as these were all sold direct from SignVideo/Studio1.)
These still sell in communities, and privately, just not eBay.