That's a great idea i think
This is way too big, i agree with what you said but they shouldn't change that much. i would change increase size to 10 or 15, with the same cost.
Add an inventory limit of X spaces (suggested 1000). Anyone that reaches above this due to the above changes would get premium boxes equating to the amount of reds you would be owed.
I don't understand what is your third point though. What are the reds?
This would be a problem to calculate what is the cost of your next upgrade with this idea though. if you bought one upgrade of 5 slots and then it switches to 10 slots, what will be the cost of the next upgrade? this could make inventory cost inequalities between players.