Leader line vs. standard line--- difference?

[Mattfish]

Just out of curiosity, I have noticed a big difference in the price of mono "leader" spools, usually sold in 25-100yd packs, vs. mono line in the larger 1/4 and 1/2 lb spools. For example, the 100 pound mono leader line is usually $6-10 for a 50 yard spool, whereas you can get a whole 1/2lb spool for not terribly much more.


Same for flourocarbon, which I've noticed is ludicrous in price (for po' folks like myself that only fish offshore on headboats).


Is the leader material more durable, abrasion resistant, stiff, supple, hard, soft? Why the big difference in price compared to what comes in the larger spools?