I've seen it several times (and while not a racer I watch a lot). I don't think it is common, feels like 5% of the time.
I've also seen Primes significantly larger than the winning prize, also 5% of the time. That really messes up a race for those paying attention.
And I've seen Cat 1 juniors downgrade to Cat 3 to take big money.
But I already posted I'm not a fan of purses in amateur cycling.
Sponsored riders tend to execute to plan, not prime, so there is opportunity to grab some cash for the others, but if it goes against plan, there is a price to pay.