Your logbook will need to contain a valid roadracing license, instructor certification, or recent and significant Advanced-group experience to get into one of those non-instructed Summit events.
VIR has their own open practice days but you need to be a VIR Club member. They're running 'em all weekend this weekend, in fact... for their members only, which is $$$. NASA runs 'em too... VIR-Full next Fri 2/23 for example... but you'll still need one or more of the qualifications listed above to be able to register.
100-200 bucks a day is cheap IMHO. As for nobody showing up, it's just the opposite... hundreds of cars at a time actually... they're very popular.
I've never heard of a road course being "open on weekends" for "free runs" and I've been around this stuff for nearly a decade now. Where do they do such a thing? I only ask because I want to move there.
Confused (and no offense intended),
Jon