why are magic dual lands so expensive

[SlowFast]

why are mtg dual lands so expensive?

[FNMfan]

well there are different ones.

the Borderposts ( From Alara Reborn ) Are acctually artfacts not lands but they tap for mana...
these also count as a multi colord permanent that activates certain cards( Esper storm blade and Grixes Grimblade) to make them bigger or stonger or different abillitiys..

There are rare lands that tap for one colorless and then if u pay one of said type of mana you can get 2 of said type of mana or a split of the two the card shows....

There is also another type of land that I beleive is uncommon that taps for one colorless but if u pay 1 life u get said color of mana to use.

there are also speacial lands that will tap for either any color mana an opponent controlsor it taps for any color but it has to be used to play creature spells or ones that are based on there Shard...( Bant has Blue ,White, Green......Grixes has Blue,Red,and Black.).. most of these come into play tapped