Net Cost: $11,000 - how much the books cost wholesale (so what JSC pays Marvel).
Then next list was how much he charges for the various covers/cover sets.
Print run for Cover C is 1,000. If he has, say 500 signed and 500 unsigned and those are only sold in sets of 3 at $80 and $110 respectively, that's $95,000 revenue ($80 * 500 + $110 * 500). Profit = $84,000 ($95,000 - $11,000) for just the 3-sets on one book which usually sell out fairly quickly.
The $200-1000 artist page rate, that was the range of page rates I remember seeing from a survey/article by Comics Beat.
I don't know what JSC's page rate is specifically but that's the range that the Big Two pay and given the circulation on books, I imagine there are very few artists making $1,000 per page. The publishers wouldn't be making any money if $1000/page was the norm. I remember reading somewhere that Marvel's typical budget per book for creative costs is ~$15,000 and that has to be divided among writer, penciller, inker, colorist, letterer and cover artist.
So yeah, JSC is making at least $80K per month doing just covers (he's both cover artist and retailer so no middle man) or he could be slogging through 20 pages of interiors for $4-20K per month. I dunno about you but given he's already built-up a following, I think doing just covers is the smarter choice.