I used to charge about $1. to $1.25 per yard of quilting - I figured I could quilt around 4 yards an hour. Figure out a unit of the quilt (block or whatever) figure how much quilting will be in that area, multiply by number of units. Similar for borders, figure out how much quilting will be in each running foot of border, multiply by number of feet. Add 10-20% to total. This way I could always give a fairly firm price BEFORE I started quilting.
Nowadays not too sure about the 4 yards an hour, I was hand quilting last night and my wrists started bothering me.
I charge less nowadays, by the way.