Thank you for disclosing this information, however even with an increase to the base chance of recieving a legendary drop, this still doesn't guarantee that you won't be able to purchase hundreds of boxes and still not have the weapon you desire.
I would suggest adding a "pity timer" mechanic, that either increases drop chance as you purchase more of the same boxes, until you hit the jackpot, or alternatively make it a guarantee that the legendary will drop upon the 100th or 150th box, and then reset the counter.