Randy I created this cost sheet a couple years ago, but it still is about right.
I run pump gas as do most drivers so 20-25 gallons of pump gas will get you through most weekends.
We race about once per month in cool season like now. There is usally at least one free weekend between races, but mostly 2 or 3 weekends. Most cars don't need any work between races. Just gas and go once the car is fully sorted.
NASA has a program that can take brand new guys with no track experience and get them racing. It can take from as little as a couple of months to a couple years to be ready to race. You start out in HPDE 1 with a driving instructor and then progress as you skills allow for in to the higher HPDE groups. All you need is a street car for HPDE. You don't need a race car for this. Once you have demonstrated an ability to drive on the track you can take a competition school which will set you up for racing.
As for spectators the bridge at PIR is the best place to get a view of the action. At firebird you can only the the main straight which is the most boring part of the track.
If you drop by at a good time as a 944 driver for a ride. Most have passenger's seats and are willing to take some out for a few laps in street car group.