Upgrade to C+ will not be a factor in online seat upgrade (FCM) pricing. As others have noted, there's no real way to know until after you book which will be cheaper and an agent will not be able to tell what the seat upgrade offer pricing is until after you book the flight. As an example, I'm seeing seat upgrade offer of $321.43 on DTW-LAX, but the fare difference on DTW-LAX from Main to F is $300. While on LAX-SEA route, the seat upgrade offer is $95.68 which is cheaper than upfare to FC from Main which is $146 ($104 vs. $250). This is with Z class availability on both DTW-LAX and LAX-SEA flights which generally coorelates with lowest seat upgrade offers.

If you are on connecting itins, it gets even more complicated. I'm booked on a single DTW-SEA fare with DTW-LAX-SEA routing and the cost to upfare the entire itin from Main to FC is currently only $100 total. While using seat upgrades on each flight individually would cost $321.43 + $95.68 = $417.11. This is because the seat upgrade offers are always hop-by-hop, while fare pricing is based on origin and destination (O&D) and does not use hop-by-hop pricing. The reason for the cheap FC fares for DTW-SEA is due to competition from Alaska Airlines. While on DTW-LAX standalone bookings, DL has no LCC/legacy competition on route and the fares are signficantly higher. This is where hidden city/skip-lagged bookings can save quite a bit of money (if you just want to go to LAX, book DTW-LAX-SEA on single fare, and just skip the LAX-SEA segment). The Delta Contract-of-Carriage forbids such intentional bookings. But generally speaking, airlines don't go after such behavior unless it is highly repetitive and obvious.