Are the prizes you get random, or set? And are they even worth it?

It has been quite a while since I collected all my prizes but to the best of my recollection all the prizes are sP. Worth it? You will not get back as much as you spend unless you were very lucky in restocking or vending. But there are other rewards that are less tangible, like achievements.

They're set. You get a set amount of sp based on the amount of beanbags and a trophy for your profile.

The prizes are an upgraded medal with some sP. It is never really profitable to collect, it's just a little gift.