What I mostly find unlikely is the kind of conspiracy that involves huge numbers of people, or stays private for long periods of time.
If your conspiracy theory involves 4 people who keep no records, then it's possible that they could all keep their mouths shut, and that you don't have someone inside the conspiracy who either develops a conscience, or isn't smart enough to cover their tracks. If your conspiracy theory assumes the existence of thousands of NASA employees, contractors, and others who have seen the plans, and must last for decades without any of them ever deciding to go public, then my belief levels are so close to zero as to be indistinguishable.
So I find price-fixing conspiracies entirely possible, but I also expect them to come out after a few years, because information _leaks_ constantly. I don't tend to believe things like "Big oil bought up the recipe for a car that runs on water and buried it." or "Phone batteries stop working after 18 months because the manufacturers want to force you to buy new phones." because they require every car/phone company to work together, as well as the entire scientific community carrying out research in those areas, with none of them ever deciding to spill the beans.
And, having worked in private companies for most of my adult life, they just aren't that competent.
Original post on Dreamwidth - there are comments there.