If the cost was significantly lower, people would not kill Glacors to get them.
When the boots first came out, they were around the price they are now. After a few days of their release, they started to get lower and lower, and as a result Glacors became empty.
This resulted in the boots rising in price again, to the price they are today.
People are only going to go through effort to get something if it is worth it. You have to Mage Glacors, so if the boots get cheap enough, it isn't worth your runes or your effort killing them.