I'd say somewhere between $500 and $5,000, but...
This beast might be upside down and in baskets for all we know. Condition is a lot more important than mileage (IMO), but we need to know both.
Here's an Ebay search of '55-'59 trucks, COMPLETED listings. Notice that most SOLD patina'd trucks are $4K and below, and some of those are drivers.
Add about 20% because it's a panel, but notice that that '57 California panel that runs, stops, drives and is "almost restored" (I don't think so) didn't sell for $9500. The blue "20-footer" panel that is popping rust also didn't sell for $9500.
People can ASK anything on Ebay, and they often do...