I had problems with the price of bullets the first time i played a gunslinger. The price is most likely because unlike arrows which are easy to find and buy, firearms and their ammunition is not the first choice for many adventurers when then get a ranged weapon. Bullet's also have many advantage's and disadvantage's. I suggest if your using firearm's look into alchemical cartridge's.
A firearm is not a simple weapon probably for the fact is, not everyone fully know about them in the world of pathfinder, and most of the people who do prefer not to use them for the fact that they don't know much about them. Firearm's are rare on adventurers, and making them simple, would ruin the purpose of allowing certain classes to use them. The gunslinger is a daring class to learn the way's of how a firearm works, and using them, letting any class do that would kind of defeat the purpose of having gun's be kinda mysterious and feared in pathfinder.