At their current state, the boxes are nothing more than pure scam. The description for the boxes aren't really helpful, no word about the dropchances of the top tier stuff ... but i think, they don't have a glue how to calculate the droprates themselfes. The possibility to get duplicates out of the boxes over and over again is making it impossible to tell.
So far, i can remember only one f2p game that had some kind of "fair" lootbox system for some time. S.K.I.L.L SF2 had a progressive system ... you bought "capsules" to get a specific skin weapon and they counted the amount of capsules you already opened with the guaranty to get it in capsule Nr. 100 (which was around 40-50 EUR).
With some luck, you got the weapon in capsule Nr. 10, 30, 50 ... if not, you knew it wouldn't cost you more than the 40-50 EUR to get the weapon which is still more than enough for a bunch of virtual pixels ...
Sadly, they got greedy again and they removed the system after some time.