Well, the rule of thumb for many artists is if it was popular then than it will retain it's value now. $500 was a huge amount of money back, especially for an American painters, whose works were generally less valued than a European artist of the time.
That's just the general wisdom. Without seeing a work in person to assess it's quality and condition, nobody can tell you over the internet what it's really worth. I suggest taking it to your local art museum who can tell you what it is.
Good Luck, G.