Spamming a max lvl High heal with max Offertorium costs about 570sp per cast! Great for supporting characters with 'critical wounds' or against mvps.
Sp conserve gear is a great way to make up for the whopping sp cost.
Some items that can help:
Heart Wing Hair Band (-10% and -3% per 3 upgrade lvls, reduction in sp use)
High Priest Skill Spiritual Thrift (-20%, reduction in sp use)
Sarah's earrings (-10%, reduction in sp use, if perfectly enchanted) can have 2
Lets list other suggestions?