Around these parts that would be a couple hundred bucks complete easily.
It's from the flight liner family and could be badged under a few different names.
Just that tank which even appears to still have the lens would fetch well over the asking price.
It looks to be a Murray made bike complete other than the chain guard and the rear fender. They are middle weight bikes from the late 50's to the mid 60's
Just look up info on the spaceliner and flight liner bikes.
It's made by Murray and worth the $40 which I think is what the tank is worth alone. The frame is the same as what Murray used for the Sears Flightliner.
Serial number should help date it if you ever bought it but it's at least 1958 to maybe the mid 60's.
For $40 you better get to grabbin' Lots of guys here would pay that just to get their hands on that crank hub cap. I have an extra chain guard if you do get it and need one.