Car Value

I have a friend that needed a place to store his old project car.  In Sept. of
2014, he had his 1970 Chevelle towed to my back yard.  It was supposed to
be picked up in spring of 2015.  Here we are in Aug of 2018 and the car is still
in my yard. I have not heard from the owner in at least 3 years and heard that
he moved to another state.  My question is, how much is that car worth?  It is
in pretty rough shape. The engine is sitting in the back seat, the interior is shot,
there are broken windows and rust.  It looks like it came from a junk yard.  I
recently had someone knock on my door and offer $500.00 for the car.  I told
him the situation with my friend and that I do not have a title.  This guy has
been stopping by and calling a few times a week and he seems really eager to
get that car.  I get the impression that it may be worth more then $500, even it
its current condition and without a title.  Can someone give me a ballpark
estimate on what the car is worth?