Instant and sorcery spells you cast costless to cast.
Whenever you cast an instant or sorcery spell, scry 1. (Look at the top card of your library. You may put that card on the bottom of your library.)
6/22/2015 The first ability of Jace’s Sanctum can’t reduce the colored mana requirement of an instant or sorcery spell.
6/22/2015 If there are additional costs to cast an instant or sorcery spell, apply those before applying cost reductions.
6/22/2015 Jace’s Sanctum can reduce alternative costs such as miracle or overload costs.
6/22/2015 Jace’s Sanctum’s scry ability will resolve before the instant or sorcery spell that caused it to trigger.