Its not too late, but I have no regrets. $500 is allot of money for anyone really, but its not really that bad on an STi.
If you think about it, the STi is actually one of the least depreciating cars on the road today. My car is my hobby. I love driving my STi. $500 a month is pretty much my membership fee for my hobby, so its really not that bad in that respect as nothing else really costs me money besides gas.
Its like someone who shoots guns all the time. Ammo is really expensive. But if its what you enjoy, its what you do.
After this STi, I will most likely back down into something less expensive as I will be planning on having kids around then. This is kinda like my chance to have a car that I consider to be a very very nice car.