Hi Michael. As PBE explained, determining the value of these old beasts is difficult (if not impossible).
The value is ultimately determined by the highest price someone will pay. We require that sellers post a price in order to reduce the animosity between potential buyers.
The weber collecting community is big, but it's still in its infancy.
There are some buyers that may be willing to spend 500+ on a grill like this, but many of us would only be willing to spend a fraction of that.
In reality, most of us are in this for the thrill of it. We're used paying craigslist junk prices and we refuse to spend anything beyond a craiglist junk price.
99% of the grills in our collections were found on craigslist for less than $50 each.
I got my lime, which was near mint, for $30.
If you want to get absolute top dollar, gently clean up the outside and inside. A hose and a gentle scrub with some paper towels is fine. Take a dozen good photos.
Post an ebay auction, start the bidding at $1 and ride it out.
We'll post the auction here, where all of the collectors are, and you will get the most someone is willing to pay.
We even have a guide to help you packing and shipping your kettle
You are missing the original wheels, and that's going to deduct value.
Luckily you still have the original ash pan. Your bowl and lid appear to be in pretty good shape.