Back in the late 90's I was put in charge of Skycaps by the airline that I used to work for. Managed a work force of over 300 of them for a few years.
Overall they are good hard working folks that do their best each and every day. They made significantly less than minimum wage because of tipping. Trust me...they made out just fine!
A decent Skycap working 4-6 hours a day (most of them worked part-time back then) would make between $100 and 150 per day CASH! A very good to excellent Skycap would make double that!
It all depended on which shift they worked (seniority got the best shifts) and how good they were with customer service.

Now that I'm retired whenever I travel I try to use Skycaps. For good to excellent service I tip $5/bag. Average service gets $2/bag.