As a high school teacher, I've witnessed a couple of these Prom-posals. They're not particularly common; in a school with a senior class of about 500, I'd estimate perhaps 15-20 guys do Prom-posals. And they're not spendy. Typically flowers are involved, but we're talking about a grocery store bunch costing $10-12. I've seen hand-made posters, balloons on cars, sidewalk chalk, stuffed animals.
A friend of mine who's the mother of teenaged boys says that this is something boys do if they're not too sure of the reception they'll receive. It's harder to say no when the guy's gone to such trouble.
The girls LOVE, LOVE, LOVE, LOVE, LOVE these Prom-posals, and they are the talk of the school. If girls were doing the asking, they'd be more common.