I can't go because I'm going to be in China at that time, but just a quick calculation:
You can get to Colorado in 25 hours. However, after around 16 hours of driving, anybody coming from DC is going to start feeling tired. So, factor in lodging money (around $50 a night per person if you play it cheap)
Also, as a note, gas is going to cost $3.50 a gallon average, not $3
A car that can comfortably fit 5 people and their luggage (while running the air conditioning, because August is f#cking hot.) is going to get the quoted 20 mpg.
So the cost is going to be (1684/20)=84.15
84.15*3.50= $294.53 for gas.
Assume $20 a day for food per person (You should be eating 3 square meals a day to keep your strength up). Driving there means you leave on the morning of the 8th, get there somewhere in the morning of the 9th and rest so you can be refreshed for the jamming on the 10th. On the morning of the 14th you leave, and get back sometime in the morning of the 15th.
You'll need food from the 8th-14.
8,9,10,11,12,13,14= $140
So personal costs are going to be roughly $140+50+58.91= $248.91
Flying is $253 for roundtrip tickets+ Food ($20*4)= $333.
So, driving is still cheaper, but not dramatically cheaper, not to mention that it takes up more time and leaves you in a cramped car with a bunch of sweating guys, and requires you to shack up in a potentially bedbug ridden motel for the night.
Just keep that in mind.