I just checked a bag at LaGuardia last weekend. It had a sign posted that the fee to curb check a bag was $2 per bag. The guy who got tipped by IzzyJG99's Mom did better than the guy who checked me in. I tipped, but he did not get $5 in all, only $4.
Although, I would've given him a $5 if I had it, mostly because I kind of see that as the normal going rate for curbside, based on past comments and experiences.
There was one guy I gave at least $20 to a few years ago, but he got me out of and around a line when I was late for the plane.
If you look at the math, these guys work for probably 8 hours a day and check in on average probably a bag at least every five minutes. (When they are busy, I think it's a bag a minute, so I am liberally figuring in some slack time.) That's at least an additional $128 a day on top of any hourly wage, with just $2 per bag. Most have to do more bags than that and get more per bag than that.