The upgrade prices almost always coorelate with fare bucket inventory availability on flight (J/C/D/I/Z for FC). If there is Z inventory open on flight, then it is almost certainly the lowest upgrade offer and you won't see anything lower than that. Also, the nominal fare difference between Main and FC on SFO-MSP is currently $100 (if you are booked on a SFO-MSP fare and not connecting elsewhere and there are no FC inventory contraints) and online seat upgrade offers are rarely less than nominal fare differences on a route.