Silent, that's the key question: buy a used streetbike vs. buy a used trackbike, assuming same year and make. Does the difference in price justify the risk of buying truly damaged goods? I don't know how to evaluate a crashed bike's frame geometry. Is this a serious concern, or am I just being paranoid? In other words, how badly do you have to crash a bike before it is not salvageable?
Well, the answer to both of your questions is: it all depends. The price difference is going to vary in every case, and so is the risk. If you can find someone who's got a good track bike for sale at a great price, and they are trustworthy enough that you can believe there are no problems that they actually know of that they don't tell you, then the difference in price could a lot and the risk small.
I think it is very difficult to assess the change in risk between buying a used street bike and buying a used race bike. There is certainly a good chance that street bike has been crashed. Every single add I've seen says never down, but then you go to look at the bike and the handlebars are bent and the side of the motor has the paint scraped off and all that...and the bikes that don't look like that, may have had the damaged parts replaced.
As far as how badly you have to crash a bike to make it unsalvageable, also depends on a lot of factors, from the exact circumstances of a crash to your definition of salvageable. There really isn't anything that isn't fixable, its only a question of how much it costs to fix vs replace, and that can go for a part or a whole bike. A bent frame may be bent back, or replaced.
But, look at how hard guys can drop stunt bikes, and they just pick it up and keep going. I would think that any damage serious enough to affect the frame geometry would be easily visible to a careful observer. If you just picture the bike in your mind, picture it getting crashed, and think about what parts are going to hit when it goes over in various types of crash, then inspect those areas.