What is the approximate value of a 68 Road Runner with a good body but no motor
The car has some rust in the rear fenders
Hard to say, it could be rusting quietly away. I doubt that the only rust is in the rear fenders, which is a big job by itself. Post it for sale.
Not sure there is a value. Only market. perhaps search and look for a similar situation, and see what they are selling for. You can also throw it out there on Ebay, and you will find the "value" real quick, which is what someone is willing or not wiling to buy. Put the reserve or buy it now at a price you would for sure take . worst case you will collect some leads an intel..
Still worth some good money, get ye to Hemmings and compare it to other Road Runners of similar condition.
That all depends on the condition, how complete it is and what options it came with.