There are much more elegant ways but the easiest I can think of off hand is this:
On enemy destroyed > set value of powerupvariable to floor(random(100)) (or whatever number you like).
Then.
If powerupvariable = 1 > spawn powerup1
if powerupvariable = 2 > spawn powerup2 etc...
This way if you have 10 powerups then with the above formula you would have a 10% chance of getting a random powerup.