For most modern games i rarely go over £100 unless it's some special edition or a limited print company release that i missed out on for switch and had to pay through ass for it on ebay. For retro stuff probably harmful park on PS1 it set me back £470 however it's not some faded shit tatty copy it's literally pristine there's no damage at all or fading to any part of it so something not often seen at all hence bidding was fierce on ebay for it.
Runner up would probably be gegege no kitaro last game on PS1 so really low print run, was about £380
It's interesting, if you adjust for inflation it's probably Resident Evil 2, which I bought on day of release in 1998 for £50 - it's the equivalent to £110 in 2024 apparently - https://www.in2013dollars.com/uk/inflation/1998?amount=50
Phantasy Star 2 was 179,-- DM, but I never had a Mega Drive.
For comparison with today's prices:
1 € = 2 DM
So 140 DM = 70 €, and to account for inflation since the early nineties it probably makes sense to triple the number, so we end up at about 200 € for each of the two games.