Based on the information provided, it's impossible to know. The price is directly tied to the condition and functionability of the trailer - not purely size & age and a picture or two. You really need to step back and objectively assess the conditon of it before a price can be validated.
While this Price vs Condition
information in (Table B) does not
specifically address Argosy prices (which are typically lower than their silver siblings) there is a lot of great information on this webpage for assessing the true condition of vintage trailers, which then can be used to determine the value based on it's condition. This criteria is the same for Argosys, Airstreams and for that matter, just about any vintage trailer.
There are also good budget figures (Table C) on the costs to repair/replace it's deficiencies to help get you started - the costs do not vary based on Airstream vs. Argosy. If your son & the seller spend the time to read and digest this information - it will help determine a fair market value.