Yeah, Duro is the cheap stuff in the blue can. Here it's $3.96 for an 11oz. can. The applicator tip is crappy IMO, spray patten is inconsistent and it takes 2 cans to do even a small box if you want the seams to stay put any legnth of time. The 3M Super 77 is around $10 for 16.75oz. I can use one can for 2 small boxes with good results. IMO it's the better value in the long run.
I'm thinking of trying a combination. Use Duro on the flat panels and S77 on the corners and seams and see how far it goes.