I think it would be incredibly hard to get good data because it is actually an international market. So some people find postdocs and permanent positions outside the USA while some positions in the USA are filled by people coming from outside the USA. For instance, here in the UK large number of positions is filled by Germans.

One way to try to estimate the chance is to look at a good number of departments and see how many postdocs they hire each year and how many tenure track positions they fill. But I doubt if this is going to be very accurate.