I have a 1988 Gloss Black 2WD Sapphire cosworth which I've owned for 14 years now. Over the years I've done loads of mods and upgrades and piled loads of money in to improving the way it drives and goes.
Ive got mine on a agreed value of £30k with my insurance company but I still don't think I could build it again for that price if anything happened to it.
Had to prove what had been done and back it up with invoices for the agreed value but I wouldn't want to not have it agreed now as I could never replace it.