Hi there. So my band will start gigging later this year. Our first show will most likely be at a high school's auditorium. The size is probably 3/4 of what you see in the following picture:

I've never performed live before so I don't really know what equipment I need. This is what I have so far:

Korina epiphone explorer bass and a 100W Acoustic amp. I know that my equipment is not nearly enough to be heard in such a large place. I don't know if I should invest in a new amp or buy a PA system. You guys are the experts so please recommend me what I need. I'm probably being optimistic, but can the equipment I need cost between $200-400?
Actually 100w may be enough. An auditorium such as this one is made with acoustics in mind, and amplifies sound so that it is heard evenly throughout the venue.
It is going to depend on (a.) What kind of music your band plays, and (b.) what do the other members of your band play? If you are competing with a guitarist (or two) blasting through a 100-watt halfstack, you'll need at least 300 watts to cut through.

I'd go with a bigger amplifier. A good P.A. system is a major investment. Of course, you're going to need one unless you do strictly instrumentals. What is your singer singing through?
"Maybe this world is another planet's hell?" - Aldous Huxley
The school should have a PA, hook up your amp to that, but I would advise you to go used if you're getting a new amp.

Ibanez SR506BM
Ashdown Little Giant 1000w
Peavey TVX 115+410
A big ass upright