Both my bikes are getting on a bit. In fact the MTB I bought for £30 about 8 yrs ago, and have no idea how old it was when I bought it... It's had new tyres, new back wheel, chainrings, rear block, chain, but front and rear mech are the same, shifters same, frame obv the same.*
The roadie was bought (sale) for £299 about 7 yrs ago, so cheap and cheerful, but decent quality (Giant). The only thing the same about it is the frame* [edit] (and handlebars/stem, saddle/stem, brake calipers)
*So, one conclusion and one question:
1) I can't put a value on them; they were cheap, but I've spent a fair amount of cash making them 'my own' over the years.
*2) At what point does a bike become like "Trigger's Broom". i.e. does the frame = my roadie, seeing as everything else has been replaced? Is the frame the 'soul' of the bike?