My understanding of this is that it's not much lower than other entry level pay, but they tend to get no raises or smaller raises because their immigration status is tied to their current employment so they can't easily switch jobs. Often times low cost H1B workers have been used to replace more senior people too. There was a big wave of this around the time of the last recession as a cost savings measure.
I'm not super familiar with data on the topic though, that would definitely be nice to see.