I haven't found a commercial truck prices site such as kbb.com, NADAGuides.com, or Edmunds.com for cars and pickups. Those three and the wannabees all stop at F-350 DRW.
There a few websites devoted to the commercial truck market, but the good ones deal only in class 8 OTR trucks. Not much for class 5, 6 and 7 trucks. Try
New and Used Commercial Trucks For Sale | CommercialTruckTrader.com
Also you can get some ideas by surfing on E-Bay Motors. Limit your search to F-550 and you'll get several hits, some with "buy it now" prices.
