The devs of Rocket League released the drop rates of items:
https://www.rocketleague.com/news/drop-rates-in-rocket-league-crates/
- Rare Item: 55%
- Very Rare Item: 28%
- Import Item: 12%
- Exotic Item: 4%
- Black Market Item: 1%
- Chance of receiving Painted attribute: 25%
- Chance of receiving Certified attribute: 25%
https://www.rocketleague.com/news/drop-rates-in-rocket-league-crates/