I’ve always been told 50% original value and reduce by a further 10% for each year old. Which would give you about £250 – assuming it started about £600.
Having said that I’ve been looking for a 2nd hand bike for my partner’s son on eBay, and prices seem to be all over the place. £300 for 2 year old very high spec Orange hardtail, and £250 for a 4 year old GT costing £400 originally!
It was £500 when I bought it in November 2009. There’s a post on bikeradar about this model (here).
soma_rich: Revolution are Edinburgh Bicycle Coop own make. Merida make all the frames for them. I’ve got a Revolution Touring bike which was excellent value for the money, shame their customer service and tech knowledge is so poor!