I would think that £20m upfront would leave him just about breaking even, although £5m a year for the next 5 years might sound good, particularly if they fail to go up this year. Another year in league 1 will come at a pretty big cost, and even if successful next year there will be the funding of a Championship club without the parachute payments to fall back on.
I would guess that the Sunderland experience has burnt SD and he will be looking at getting out before he starts losing big money. On the back of Eastleigh costing him a fair bit without real success, I would have a few concerns about any long term financial security if he came back home.
A nice guy who has helped us many times before, but maybe a little naive when it comes to running a football club.