Very cool. I don't suppose you'd be willing to make and sell these? Might be interested in some for my brown Treks, if they ever arrive...
I can't say I wouldn't entertain a group buy offer or anything but my estimate for doing my own freehand is about two hours per pair. ~30 minutes to cut the leather, ~30 to cut the velcro, and ~60 minutes of sewing for a pair. My current sewing machine really isn't up to the task so it is mainly useful to place holes in the leather for hand stitching.
If I were doing a moderate quantity, I'd build a cutting template for each size needed, construct a hold-down jig, and then make a significant investment in an industrial grade sewing machine. After shipping, taxes, materials, consumables, and labor break-even would be $15-$20 per pair from my living room on the weekends. I am not sure how much of a market there would be at that price, but wouldn't turn down feedback on it.
At the other end of the spectrum would be higher end custom work. Shark, ray, gator, ostrich, etc accents to really show off, but I don't know how many people would spend more on a strap than they would Treks.