I`ll add to what Thoazol said using examples I have observed, all are covered, but some elaboration in my perspective.
90 Tokenz Promo (I have never seen anything cheaper)
Increased chances for certain Crs. It is mostly Big 5, but it applies for Gold Chests. The closest example was in December when #Cannibal Jo Cr was in the Bronze Chest.
Otherwise, not much to say, but it is as you said. The only promos I`ve see are boosted big 5 chance or cheaper Tokenz chest.
Personally, I open boxes when they are cheaper (90 Tokenz) as I believe this being an efficient way to get Collectors in a completionist perspective, though it is an approach I recommend for Bronze or Silver because those Tokenz are easy to get and the odds of getting the same Cr in 2 different boxes is low in my opinion.
Q: Do the bronze silver ever get cheaper?/
A: Yes in certain promos. The cheapest I`ve seen for these chests is 90 Tokenz. (Staff called it the 10% off discount on Tokenz chests I believe)
In this case, all Bronze, Silver and Gold were 90 Tokenz for a limited period of time.