Bill wrote: "Given the same pound test, what is the difference in Berkley®
Vanish Fluorocarbon (or any fluorocarbon for that matter) sold as "Leader"
material in 15 yard spools for $20 and Berkley Fluorocarbon that is sold on
bulk spools of 300 yards for $12?"

The price

It is market that way because some people do not want to spool their reels
with fluorocarbon but still want to use a little for a leader. The spools
are smaller so they take up less room in the tackle box. It is part of the
marketing strategy to make you think it is specific designed as leader
material.

If you want to buy in bulk and keep the spool at the house and carry some
for use as leaders try wounding some around a thread spool. There is
usually a small slot to slip the end over to keep it from unraveling in your
tackle box.

Sarge