I am surprised you received La Quica in a champion chest. To me, the rarest sicarios to receive "free" from the game (without spending gold) are Butcher, La Quica, and Scorpion. After 7 months of regular playing I have received none out of free chests despite winning every league level and numerous gold, champion, and other chests. I have receive the following contracts for each Sicario, Butcher (9/50), Scorpion (9/50), and La Quica (9/25). These contracts seem to be available only in bronze chests for me. Almost all of those contracts were received in the first month or so of playing the game, so almost no contracts for these in the past 6 months.
I do have La Quicas which I received through the special chest offers. Occasionally there are special chests in the store that seem to have better odds of receiving La Quica. There was one a while back that seemed to offer a 1 in 9 chance although the gold price was higher than a traditional gold chest (it was probably the "valuable" talent offer). When you click on a chest offer in the store it gives you an indication of the sicarios being offered and you can get an idea of the likelihood of receiving the sicario you desire.
The funny thing is, I never receive a gold purchase offer for la quica or a builder (maybe when I started? ), yet new players can buy gold and receive a la quica and a builder for $4.99.
Good luck.
