I can't check out the save (I'm at work), but what is "way too high?" Consider that, unless you can wipe out Persia quickly, they will most likely bring in allies. Anyone you can buy off will cease to be an option for the Persians to ally with.
RoP betrayal does horrible things to your reputation. Too bad about that. Not much you can do.
You're 1 tech from Military Tradition. Ok, fine, fight defensively (maybe bring some Knights up to the front to help out) until you have the tech for the upgrade. During this time, you should be able to engage (and hopefully defeat) the Persian attack force. Thus, when you counterattack with Cavalry, you will face minimal opposition.
-Arrian
p.s. try really hard not to lose a battle to an immortal.