The 40,000 mile price pre-purchase is based on fare difference between main and FC at 1 CPM (was that one-way or roundtrip?). The post-purchase seat upgrade price (FCM) does not use fare difference as a factor and can be higher or lower than fare difference (the mileage is based on cash seat upgrade offer price at 1CPM less 7.5% US domestic excise tax which is not charged when using miles. Formula is Miles = Cash (dollars)/1.075 x 100). USG views miles as a rebate on travel upon which 7.5% tax has already been assessed -- so charging tax with mileage purchases would be a double tax. Note that when you buy miles directly from DL, they charge the 7.5% excise tax on the purchase as that is not a rebate on travel.

With an ExpertFlyer account, it's fairly straightforward to calculate nominal fare differences on a route by looking at fare tables. Here's DTW-RDU as an example using roundtrip V fares (higher class fares all tend to have same differentials). The second to last character in fare basis code indicates class of service. M=main, I=C+ w/S class, C=C+ w/W Class, F=FC (books into Z class with V basis fares). This assumes there is Z bucket open along with V (which is why it is a "nominal" difference). If there is only I or higher availability on a flight, you'd be bumped to a higher FC and fare difference would be greater. While you can calculate fare differentials entirely from knowledge of fare class availability, fare filings, and flight dates, the FCM seat upgrades offers are all Delta secret sauce and there's really no way of knowing exactly what they will be until after you have purchased flights.

Main cabin = $164 each-way

C+ with S class = $214 each-way ($50 upcharge)

C+ with W class = $224 each-way ($60 upcharge)

C+ with Z availability = $264 each-way ($100 upcharge)