I have been the referee for two articles at an academic journal. It said in their agreement that for three or more papers per year you'd be compensated this and that much. But I guess I misunderstood because they emailed me and asked to pay me for just the two reviews. Anyhow, it basically no money. The time you put in to do a proper review is a lot more than what you are compensated for. Your uni still pays your salary, so this is just a bonus, but still, very little. This journal is hosted by a public entity, private ones may be very different.