It seems that StatTrak variants are all too rare, and sometimes hard to find. Compare this with TF2, where Strange weapons are aplenty, and unboxing one doesn't take too much effort.
However, nothing in CS:GO seems to suggest any sort of "percentage" of unboxing a StatTrak weapon. Is this a factor that is affected by the item quality that a gun gets? Is it a "set" flat percentage on all weapons in the crate?
I haven't seen any official documentation listing the official percentage (however, a "Unusual" item is almost always a 1% chance). What are my chances of unboxing a StatTrak?