just went thru re-hire myself and here's my 2 cents. Although not IT, I worked at a place with the largest IT enterprise known to man. Global secure network. Lots of high performing ITs made $75-$85/hr.
1. Ask them to make an offer, they'll lowball by about 15% I expect. Nogotiate up by 10% (they win, you win).
2. Ask for a flexible schedule of your choosing.
4. Performance bonus.
I was making $175k per year (salary,bonus,COLA, housing), when it came time to negotiate my consulting rate I aske for $115/hr, they didn't flinch. Accepted immediately and like you I wonder what they would have paid.