When CAC < LTV, but the trick here is pre-revenue so the only way you could actually really have a comprhensible gauge on this would be if you did a kickstarter, indiegogo to measure "interest" before engaging in real "physical" product development.
Remember a crowdsourcing site goal is not just to fund a product but also can help as a tool to measure "interest". Think of it as a "surveymonkey" for physical products.

"X" amount of interest could actually give you a hint of what the possible outcome could be and if you are starting at an ilussion or not.

Good luck :)