Power-Ups have to be relatively uncommon.
Their probability should be measured in the percent of chances of a power-up spawning per second.
Use "Cooper Accuracy" as a point per second estimate (2 shots per second)
Power-ups should not be found consecutively, either within a set amount of time or points. Maybe 15 seconds, or 50 points.
Power-Ups should be less likely when the score is low, and should progressively get more common as the score goes up.
Consider doing no power-ups before 100, and doing a quick one at 100 to show players the existence of power-ups.
After every increase in difficulty, make them more likely (10 or 20 percent?)
Coin Magnet & Multiplier
When Coin Magnet & Coin Multiplier are activated, the amount of coins spawned should increase exponentially.
Due to the game-altering mechanics the Death-Laser might introduce, a lot of thought might be needed to implement the feature.
One idea is to make it a 3-part power-up, where the player has to collect the power-up token three times before it is immediately activated.
The first token should appear rather early, maybe between 200 and 250 points. However, it might be best if it is not fully activated before it is closer to 500 points.
The Laser should last somewhere around 3 to 5 seconds, and give the player a total of 100 points.