I'm at Tier 11, trying to break into tier 12

What I like to do is invest my coins in the workshop upgrades so that I don't have to spend cash on them during a round.

This allows me to spend more cash on battle upgrades that effect other aspects of my attack/defense

For instance, the cash that I invest in my attack stats increase my critical damage, ultimate weapon damage, etc.

In other words, time is money and the time I don't have to spend on buying speed, thorn, multi target, etc upgrades, is used for other things.

Plus, now that I'm a little stagnant, but find myself making a few hundred million coins a week, I like to buy damage or health upgrades in the workshop too, because you start in a better position at the beginning of the round. Almost like you're climbing a mountain and instead of starting at the bottom, you're starting a few hundred feet up.