steel density 7.85
titanium density 4.5
aluminum density 2.7

(approx, depending on alloys)

So, going twice as thick with the titanium, will be heavier than steel. The Titanium Joe site had a couple of thicknesses available.

There are a couple of Titanium alloys, but they should be similar. 3Al2.5V is perhaps the most common. There is also a 6Al4V. The 6/4 is stronger, but more expensive and harder to find. Nonetheless, the metals being used for the stems should be similar whether you buy them from BF or a metal supplier. And a straight tube, no welds or added parts should make it pretty straight forward.

I don't know about prices. Both metal suppliers and bike stores have markups. But, BF should be able to get pretty good prices on raw materials.

There will be some minimal machining on the posts. Cutting, rounding edges, polishing, etc. But, it can't be that much.

Bike Friday also has to support the business, employees, and turn a profit, so nothing can be given away for free.

Some of the internet suppliers are also selling off remnants. It is quite possible that bike manufacturers have troubles acquiring remnants. One could get them from companies like Boeing, but perhaps it is hard to get a contract for just certain useful sizes.