The offers at the end before booking are not actually online seat upgrade offers. They are just upfares where you are paying the fare difference from your original selection with either cash or miles (at the fixed 1.0 cents per mile standard rate). This is easy to demonstrate (see below). You actually need to go through with the booking to see what the online seat upgrade offers will be. Agents are largely clueless on these topics. I would just book a Main seat, and not C+, as it will likely not make a difference in online seat upgrade pricing.


Notice fare differentials from C+ to PS and D1 are $306 and $1106, respectively, on the flight below. $1430 - $1124 = $306 and $2230 - $1124 = $1106,





Here are upgrade offers at booking time after selecting the C+ fare. They are clearly just upfares based on fare differentials. They are not the same as the online seat upgrade offers which require actually booking the flights first.



I went ahead and actually booked a Main Cabin fare on this flight for $780. Notice the fare differentials from the $780 Main fare to PS and D1 are $650 and $1450, respectively.



Here are the online seat upgrade offers after booking a Main Cabin seat on this flight. The PS offer is actually slightly more expensive than simply upfaring at $697.68 vs. $650, while the D1 offer is somewhat better than an upfare at $1180.35 vs. $1450. The mileage offers are a bit better than 1.0 cents per mile due to the 7.5% US Excise taxes charged on the cash offers on domestic flights which don't apply to mileage upgrades (you effectively get 1.075 cents per mile in value on domestic upgrade offers). While the D1 offer is somewhat better than an upfare, most wouldn't consider it much of a deal and OP's 94K miles would not be sufficient to pay for 109.8K mileage offer.