There are at least three versions of the 520 casting, representing the "30" series (early '70s), the "40" series (early '80s) and the "50" series (late '80s/early '90s). The castings are very similar, but the stripe on the hood is different in each case.
Some prices from Dick's Farm Toy Price Guide: "Good" means restorable, no casting breaks, "Excellent" means either almost brand new or restored.
All three versions are listed at $125 +/- for Good and $150 to $175 for Excellent.
I have scanned a couple of pages from a farm toy collector guide and can e-mail them to you. My e-mail is open, so if you want them, drop me a line and I'll send them to you.