I always give my opinion of equivalent replacement. What did they value it at last time? and what if any improvements have been made, added to market de/appreciation over the last 12 months? For my old tintops it's simple replacement value of a few hundred to a few thousand,for the Mog, I wouldn't necessarily drop below purchase price, unless market forces dictate otherwise vs probability of having to sell? They're only ever worth what someone's willing to pay for them in money terms, but always priceless to their keepers!!😁
The market value and the rebuild cost are quite rapidly diverging.
The market value of my +8 is probably somewhere between £33k to £38k (but as above it all depends upon someone being prepared to pay that) but the rebuild cost (which you can estimate more accurately) is MUCH higher at around £45k if it was to be undertaken by a dealer. I have therefore insured mine at the latter, and got my dealer to note this on the valuation form. It does cost a little more to use that figure, but the peace of mind is worth it to me.