Instant magic effects have 0 duration in custom spell cost calculations unlike vanilla
Magicka cost/spell chance of a spell with Open effect as shown on the spellmaking window
OpenMW:
15/65 Touch 50
30/50 Touch 100
22/58 Target 50
45/35 Target 100
Morrowind:
30/48 Touch 50
60/18 Touch 100
45/75 Target 50
90/30 Target 100
Ondusi's Open Door costs 15 Magicka in both, but it's a special case because premade spells sometimes have higher or lower Magicka cost than what you'd get if you'd make the exact same spell yourself with spellmaking.
Edited by Alexei Kotov