(Dragonskull Shelter isn't a spell.)
Dragonskull Shelter enters the battlefield tapped unless you pay .
: Target player exiles a card from his or her graveyard.
: Add or to your mana pool.
Sacrifice Dragonskull Shelter: Add or to your mana pool.
.
: Target player exiles a card from his or her graveyard.: Add 
or 
to your mana pool. or to your mana pool.
