Next to the plasma donation center in my town, there's a children's trampoline and activity center. I take my son there at least once a month, sometimes more often if the weather’s bad.
While he plays, I sit near the front windows and keep an eye on him while also looking outside and seeing the various individuals who go and sell their plasma at the donation center.
There are people from all different walks of life. You've got people walking in from god knows where (my town and this area in particular are not very walking friendly), people driving up in trash cars about to fall apart, people driving up in $60k+ cars, folks with multiple children in tow, etc.
It seems that many people other than the folks we'd normally consider poor need to sell their plasma to get that bit of extra cash.