If you wanted to create a prototype of an object with a large build envelope, say the size of a garbage can, what would be the most economical way to do it? Comments please!
Impossible to answer without details. Is it a Chair? A robot ? An R/C submarine ?
Depending upon what “it” is, traditional fabrication would probably be the way to go, imho if your tolerances are loose, and your object is large, make it out of wood our metal. If you need precision our fidelity use 3d printing.
All that being said it is possible to print such an object with things available on the market, but it would be crazy expensive.
The best and cheapest would be hands, if there is some delicate accuracy issues, then go to cnc milling, printing would be my personal last choice. BTW, still today human hands finish tooling for molding. IMHO, still the ultimate tool known to mankind. Hope 3d printing doesn’t make you forget that!
Lets say it was plastic, abs would great and the size and complexity of a pelican case but about the size of an outdoor garbage can. Sorry I can’t be mod specific but patents have not been filed yet. 
And thanks to all so far for the input! I am very familiar with rp tech in general but doing something this large presents challenges I am not experienced with so any thinking (inide or) outside of the build envelope so to speak, is appreciated.
Id just ad if there is quantity of one or two go for hand work, for more turn to cnc router shop. Will be far cheaper than printing and this way you have tons of materials you can work with.
You could make your own printer, scaling up a delta printer would not be too hard, but it will take FOREVER to print. And you better know exactly how much material you will be using because if you run out 10 hours into the print you will be sol. For one print alone will be greater than $150. IF the print is successful then I really hope you lamination is good, or it will fall apart. This will not be a quick cheap or easy project.
Purchasing a printer for something like this is expensive, objet makes one with a build area to print a bike, but it is like 2,000,000