Several years ago, I had a van that the (other person's) insurance
company wanted to total when the repair estimate was $2800; they offered
me $2000.
I talked with an adjuster from my insurance company who told me the
insurance companies subscribe to an independent service that keeps track
of sales for this purpose, including newspaper ads and auctions. He
checked the database and came up with only 3 examples of 1978 Ford Vans
in the previous few months in Portland and Seattle, prices ranging from
$1600 to $2800 - and no information about them re mileage, equipment, etc.
I went out the the used car lots to see what a) a comparable van would
cost me and b) what could I get for the $2000 offer. Answers were a)
about $4500 and b) junk.
I ended up keeping the van and getting $3000 from the insurance company
(without having to accept a restriction on the title).
So what I learned from the experience is to be a nuisance when
negotiating with these people, keep track of advertised sales, be ready
to compare - especially the "can I get something comparable for your
settlement offer", and don't rush - especially if you are driving a
rental they are paying for.