I bought and used 2 600ft kits for spraying ductwork on my house. The thing you need to remember, is those kits are rated at the MAXIMUM possible coverage under ideal conditions. You will not be spraying in ideal conditions. the coverage will be less.
I think kits can be an effective solution if you have issues that prevent a contractor from spraying efficiently. In my case, i was spraying ductwork in my attic buried in cellulose, and i was not able to uncover all ductwork concurrently. Kits made sense because i could work slowly and carefully, which a contractor won't want to do.
#1 rule of thumb: You cannot purchase materials to spray foam cheaper than the contractor can complete the job for.
The contractor price should be about a $1 a board foot. That's installed. In my experience, having done a full house for over $10K in spray foam, you can't get it done any cheaper than hiring a contractor if you have a building ready to spray and they can get at everything easy.
And i looked at every possible way to cut corners including buying equipment and even MAKING my own equipment to spray foam sets. There's just not many shortcuts. My experience anyway.
