Bookfinder.com is owned by Abe Books, which is now owned by Amazon. The first non-Amazon (or Abe) US listing I found for that book leads to superbookdeals.com, where it ended up being on sale for $28 with free US shipping. I couldn't find immediate evidence that that seller is also owned by Amazon, but if it is, you could look at Alibris.

It does seem like a weird game that Amazon has ultimately acquired this service for discovering competitors, then left it operating.

Though you were actually talking about quantifying the value of choosing alternatives to Amazon, I thought it was worth pointing out how to find some other alternatives.