Você está correto, aplica resistência primeiro e depois metade do dano. Isso é declarado no Compêndio de Regras, e as entradas de entradas de glossário de MM2 e MM3 dizem quase a mesma coisa.
Compêndio de regras p226 (ou MM3 p217 e 218, ou MM2):
Half Damage
Some powers deal half damage when they miss, and some effects, such as the weakened condition, cause damage to be halved. When a power of other effect deals half damage, first apply all modifiers to the damage, including resistances and vulnerabilities, then divide the damage in half (rounded down).
Insubstantial
Some creatures, such as wraiths, are naturally insubstantial, and some powers can make a creature insubstantial. While a creature is insubstantial, it takes half damage from any attack that deals damage to it. Ongoing damage is also halved.