Cost of attendance is your whole budget. You can get your $20,500 in Staffords followed by Grad Plus loans up to your cost of attendance. In other words, you can get as much grad plus as possible up to your CoA minus other aid. All this means is that your CoA-$20,500 is your grad plus limit unless you are getting scholarships, grants, or other aid. Then it would be CoA-20500-other aid is the amount of Grad plus you can take out.
FYI, private lenders are no longer giving out above the CoA loans so you might as well stop looking.