Rollback between castles during auctions.

I wouldn't want to share such useful information. I'll just say that the rollback depends on at least 3 factors:

1. The relative strength of the castle on the map.
2. Who the opponent is and what their preferences are.
3. What roads are on the map (fast-medium or only medium).

Based on this, there won't be a single price in a similar trade against different opponents.

P.S. For Jebus, these factors don't matter.

Well, it's clear that a champion wouldn't share information. Although he gladly commented on videos showing how to take a Necropolis. ;)

It's clear that there are many factors, but I've often observed situations where the opponent simply overbid and lost because they couldn't stop (I still set a limit for myself and trade up to that limit).
The opponent's preference is a factor, but good players play more or less with all castles; they might just get demoralized because they weren't allowed to play with their favorite.
And roads are not such a significant factor; I think it affects 200-300 gold at most.

In fact, we are evaluating the 1st point here, which is why I created a table, which can also be expanded if desired.