Chances to get Tier 8/9 Tanks:
Gold Per Box depending on the total number of boxes opened:
Total gold depending on the number of boxes opened
I got the data by simulating opening x number of boxes 500,000 times.
To estimate the bonus gold drops I use the observed results here:
But multiplied the percents by 1.01825 to get them to add up to 85.94
Source: https://www.reddit.com/r/WorldofTanks/comments/187rpft/holiday_box_analysis/