If you want it close to the mountain, its really hard to find a legal parking spot you can park at all day for free or cheap even. They let you park on the frontage road in front of the parking structure after it fills up, but if you try to do it when its not you'll at least get a ticket if not towed.
If its a weekend, I recommend the Vail Mountain School parking lot and a free bus ride to the mountain. Not sure their current policy, but as long as school is out, I don't think they mind too much. Other then that, there really isn't a great option anymore.