Take this for what it's worth - it's one person's experience.
In 1988, in a large major hub in NJ, PT supervisors started at $1400 per month. Raises were at the discretion of your manager or division manager and it wasn't uncommon for a merit increase of $200 / month after 6 months, if you were well liked. My FT hub sup at that time made about 3600 a month + MIP. PT sup was a salaried job then, theoretically it was based on "5" hours but in a large hub many PT sups worked 7-8. All management was paid on the 15th and 30th of the month.
You couldn't complain to FT mgmt about hours, because to hear them tell it, they worked 14-16 per day, so 7-8 sounded like part time to them. Rumor had it that the MAX of the PT supervisor pay scale was $2625 a month then, but you had to have been there for years to get that. Rent on a nearby apt was 550 a month, so you could get a roommate and afford a car even, on a PT salary.
New PT sups at the same hub in 1998 were starting at $1375 a month. Just like the hourly wage, nothing had changed over those 10 years.
I don't know what the rate is for a new PT sup is now, I'm too far removed, but if you put the inflation calculator to the $1600 per month in 1989, it comes to $3105 a month today. Of course, PT sups now are paid on an hourly basis, with 5.5 per day and 27.5 per week guaranteed. Hours between 27.5 and 40 are paid at a 10% premium, and hours over 40 are paid at time and a half.
Jim Casey believed you paid people as much as you possibly could, to get the best people. As far as I can tell, UPS still pays its people well above the industry standard. Having said that, there's a general sense that now that UPS is public, there has been Wall St. pressure to reduce the size of the pay package to industry average, as Monkey said above me.