By upgrading your boosters you actually make more I get that when you play a lot of battles you lose the boosters quicker but thanks to upgrading them to the highest lvl it got me more credits than not upgrading my credit boosters so it kinda depends on the person choosing if they want to upgrade it or not, but I prefer upgrading
If you upgrade your boosters ye you will get more money per booster, but it depends how many you have and if you upgrade them all the way you will lose a lot if you had a generous amount, and if you do the math some times it’s better to have more of 1 then less that earn more. ;)