Titanium tubing isn't that expensive by itself, a lot less than $100/kilo. The bike would have roughly 2kg of material, maybe less. So the raw tubing material cost is not so huge that it would be unrealistic to envisage a bike frame at a reasonable cost.
It's hard to work with, about the same as stainless steel. It's hard to weld well.
So if one assumes that the machinery is up to scratch, especially the welding setup, then you're good to go (also assuming you have experienced operators).
So I don't immediately see why a folding bike at the $1500 price point is impossible. Initially I thought it was a pipe dream until I looked at bulk titanium tubing pricing.