Would you need a reservation for a typical weeknight?

Unfortunately, Red Rocks doed not take reservations except in the group camping area for larger parties. It does fill up often even on weeknights, best time to drive through and check seeems shortly after checkout which is 11am. People often drive through earlier to see who is checking out that day by looking at receipt which is posted at each campsite.

How much? Are there any spots available?

Like for any good, popular campground, plan to arrive early to pick a good site. If you come in late, they can accommodate you in the walk-in tent sites, or camp in the parking lot for the walk-in tent sites which is what we did on our first of 2 nights.