Jace's Sanctum
Enchantment
Instant and sorcery spells you cast cost 
less to cast.
Whenever you cast an instant or sorcery spell, scry 1.
Adam Paquette
Magic Origins Promos (PORI)
• #61pRulings
The first ability of Jace’s Sanctum can’t reduce the colored mana requirement of an instant or sorcery spell.
(wotc • 2015-06-22)
Jace’s Sanctum’s scry ability will resolve before the instant or sorcery spell that caused it to trigger.
(wotc • 2015-06-22)
If there are additional costs to cast an instant or sorcery spell, apply those before applying cost reductions.
(wotc • 2015-06-22)
Jace’s Sanctum can reduce alternative costs such as miracle or overload costs.
(wotc • 2015-06-22)
Rulings
The first ability of Jace’s Sanctum can’t reduce the colored mana requirement of an instant or sorcery spell.
(wotc • 2015-06-22)
Jace’s Sanctum’s scry ability will resolve before the instant or sorcery spell that caused it to trigger.
(wotc • 2015-06-22)
If there are additional costs to cast an instant or sorcery spell, apply those before applying cost reductions.
(wotc • 2015-06-22)
Jace’s Sanctum can reduce alternative costs such as miracle or overload costs.
(wotc • 2015-06-22)