The probability after reattaching a tx shouldn't change, assuming that the node you use is not overloaded and has good neighbours. However, the cumulative probability does increase of course. Please don't spam reattachs through, just wait 30 minutes.

Taking a look at my transaction history, almost half of my transactions went through without having to reattach. A single reattach fixed this in all cases except for one.

There are some numbers on this page, don't know how accurate they are through http://analytics.iotaledger.net/stresstest.table