I did not know either so I checked on google and this is what I found:
Only the finest jeans are made of selvage denim.
Selvage denim is made on old-style shuttle looms rather than modern, projectile looms. In simple terms, this means during the fabric weaving process, the cross-thread goes back and forth as one continuous thread, rather than as individual threads for each cross weave. As a result, selvage denim has a clean edge. Modern, single thread weaving has a frayed edge.
So that would explain the high price of them.