There are no guarantees the upgrade offers will be less than simply paying fare difference. If you think about it, if this was always true and predictable, it could potentially cannibalize their FC fare prices. The main differences of the online upgrade offers is that you can upgrade individual segments on trips that involve a connection (not possible on a upfare as generally all segments in each direction will be on a single fare component), and they don't touch the underlying ticket (not an issue if you buy your own tickets, but it can be important for people on agency issued tickets when flying for work).

I just flew UA roundtrip on DTW-MSP last week and the upgrade offers were consistently $80 for DTW-ORD leg and $90 for ORD-MSP in each direction up until a week or so before flights. The nominal fare difference between Main and First on UA DTW-MSP fares is currently $130 each way if you upfare at time of purchase (it can be more if there is limited inventory in FC fare classes, but outside of that it should generally be $130). So clearly if one wanted to upgrade both legs in a direction, the $130 upfare option is a better deal than paying $170 for the online upgrade option on both flights. But if you only want to upgrade one leg, the online upgrade offer would be a better deal since you can't just upfare a single leg when there are multiple legs on one fare component (you can upfare each direction independently, since a roundtrip has separate fare components each way, but not individual flight segments).