Well, let's see....
SR 13 costs, at a minimum, something like 9,153 gp. If you're spending about 1/4th of your wealth on your Armor/shield for this purpose, you're looking at about level 9 or 10.
A 9th level caster (without anything boosting spell penetration) will get through that on a roll of 4. You have a 15% chance of not being affected by that Empowered Fireball.
SR 15 costs, at a minimum, something like 16,153 gp. Again, at 1/4 of your wealth, that's about level 11. An 11th level caster (without anything boosting spell penetration) will get through that on a roll of 4.
SR 17 costs, at a minimum, something like 25,153 gp. Again, at 1/4 of your wealth, that's about level 13. A 13th level caster (without anything boosting spell penetration) will get through that on a roll of 4.
SR 19 costs, at a minimum, something like 36,153 gp. Again, at 1/4 of your wealth, that's about level 14 or 15. A 15th level caster (without anything boosting spell penetration) will get through that on a roll of 4.
A Very Old CR 18 Black Dragon, on the other hand, only has a caster level of 9.....