Just supply and demand - there's maybe a hardcore of a few hundred people who quite like the Sapphires now and they're probably going to hang onto them. If they don't there'll be a few people who maybe want one which will lead to the prices staying high for a 4-door Sierra. So many got broken for spares/crashed/rusted it was obvious it wouldn't go on for ever

For me it was always the 'best' road car - I could have bought an older 3-door for much the same price when I got mine but just preferred the 4-doors, not having the big wing on the back and the different handling characteristics