Money
The parking itself may not be the expensive part.
Red Rocks feeder page
Red Rocks parking cost: free does not always mean easy
Direct answer
Red Rocks parking is usually free, but that does not make it the best plan. The real cost is arriving early, choosing a lot, walking at elevation, leaving with traffic, and having a sober driver.
Friction
The harder parts are timing, traffic, walking, and getting out after the show.
Default
If your group wants to drink or avoid the logistics, shuttle is usually the better choice.
Recommendation
Drive only if you want control and have a sober driver. If you are trying to remove stress from the night, compare the shuttle plan instead.