I've been lucky over the years I've gone 5 or 6 times, I just show up and buy a ticket at the box office when I get there. If you ride a motorcycle, parking is also free! In prior years I've had seats in the Grandstand, Turn 1, Turn 15/16 and general admission. Very rarely does it sell out completely so it's more how much you want to spend. Although it's a hike to get to the top of the hill ( you get your cardio activity in), Turn 1 is fantastic to see the long stretch and then tight bunching around the almost 140 degree turn. Turn 15/16 is great to see the backstretch at full speed, full braking and lots of turns with the bikes fully inclined over. One year I went with my wife and son and bought tickets in advance on Turn 15/16, so we could guarantee a seat since it was their first time.
If you go by all means pay a few bucks and ride to the top of the Red Tower...it's an amazing view from the top!