Eventerke and Carerra are spot on. Going to an AX and watching is like going to the prom and not getting a kiss goodnight. You want to get out there and try it, but will end up frustrated or bored if you aren't part of the action.
You mention $$ for AX set up. Short of a full tank of gas, tire pressure gauge and lunch money, you are pretty much ready to roll.
That said, here are some simple tips that will help you prepare.
1. Completely empty the car. Old McDonald's cups, CD's, small rocks, dead rodents, last year's final exam, cell phone charger, the phone number of that girl you forgot to call....get it all out of there. Next, remove the driver side floor mat (your feet can get hung up in it on an AX course). Wipe down the dash, steering wheel, and shifter with a clean dry cloth. A lot of people who clean their car or take it to be cleaned use products like armor all to make things shiny inside. It works, but it also makes thing slick. Not a good idea if you are steering in an AX environment. Also, the glare off of the dash tends to reflect onto the windshield and compromises your vision.
2. Wash the car. Clean cars go faster. Simple fact. Like above, do NOT dress the tires with Armor All or similar products. Tend to degrade tires and while it is fresh, make things slippery.
3. Scope out the lot where the event will take place. Locate gas stations, convenience stores and the like. Gas stations have air pumps for you to set your pressures without having to drive too far to the event. 10 psi over the factory settings on all 4 corners. This will strengthen the sidewall and help with cornering. Like Eventerke said, be sure to return the pressures to stock after the event. Over the course of the day, you may want to change pressures, but since this is your first event, I would leave them there.
4. READ. If the sponsoring body has a web site, visit it frequently. If they have stories posted or a newsletter online read them. This will help you set expectations. Number of cars, culture of the organization, rules at an AX and it will also familiarize you with AX terminology like "off course", DNF, car classes, prepared, modified and so forth.
5. Dress Code. Though not required, wear lightweight long pants like sweats or cargos. I find that I slide in the seat less. Lightweight shoes are great. Tennis shoes are your best bet, but leave the hiking boots at home.
6. WALK. On the day of the event, walk the course as frequently as you are permitted. While doing your walk through, ask questions of the other drivers. Ask for tips. If you don't understand something, ask them again. No one will think you a fool. They will have better respect for you if you continue to ask. The only stupid question is the one that never gets asked.
7. WORK. Buddy up with a veteran on the day of the event and work a corner. This will give you another opportunity to ask questions and see the lines that the other drivers are taking.
8. RIDE. If possible, ride with a veteran. Depending on the event, most will have no objection to taking you on a ride along.
9. Finally, take an instructor. Take your time when you are on course and let the instructor teach you. AX courses have a lot going on in a very short period of time. Having someone in the right seat does you no good if you tear out of start and over drive the car. Listen to them, but drive at a pace where you can actually put into practice what they are advising you.
10. HAVE FUN. DON'T TAKE YOURSELF TOO SERIOUSLY. DON'T SWEAT MISTAKES. DON'T FEEL LIKE YOU HAVE TO PROVE ANYTHING