@dlrg - as you well know, MYB doesn't reflect other aspects of the individual medal: was he/she a casualty? the named unit? Rank? supporting paperwork?
And-more importantly, probably-who is in the room/online on the day. I've seen a fairly standard Gulf 1991 with clasp go for North of £1000: turns out it was two family members bidding against each other. The Red Mist descended and away they went . . .
You're asbsolutely right FourZero. There can be innumerable factors involved. In all fairness to MYB, they do only claim to be a guide.