50% of price plus 10% per year isn’t a bad yard-stick for mainstream stuff, Spesh, Giant, Trek etc.
More ’boutique’ stuff seems to be a complete lottery, either it’s in vogue and seemingly immune to meaningful depreciation – ‘custom build’ Bronson for more than the same or better spec SC full build – yeah why not. I’ll take a worn drivetrain, a few marks and no warranty over the dreaded ‘Single C’ frame. Or falls like a stone when the bike that only 4 people outside of North America cares about is looking for it’s second owner.
At the other end of the scale, YTs a ‘discount’ bike co, Capra especially, £2400 at the end of the season, £2400 on eBay the same day – sold.