I'm guessing you're booked on UA1444-UA1198. The $202 upgrade price is the present fare difference between H and P, so that's the standard offer. On the MCI-ORD flight they may sell a few last minute first class seats to some business person who needs to be in Chicago Monday morning, then give what's left as free upgrades for elite/business travelers, so I would not expect a different price for that leg.
On ORD-CUN it's less likely to have business travelers buying into first, so UA may cut the upgrade price for the last 6-7 Business Class seats for sale (J7 JN6 C6 D6 Z6 ZN6 P6 PN6) at check in to lure vacationers interested in splurging (present H->P fare difference is $119), but then you're competing with a full plane of people, some of whom may get lower offers because they have different fares. Plus, once you're checked in from MCI they may not offer you a different upgrade price from ORD, or at least you'll have to go look for it.
If $404 is worth getting free checked bags, Premier Access, avoiding a full 737-900 economy cabin (not a very comfortable place to be), enjoying the United Club for your long layover in ORD (the renovated ones in the B Concourse are nice, the one in C isn't, if it's even open), and thus feeling like you're starting your vacation early, might as well take this offer. If you are fine with your economy seat assignments and can think of better things to do with some or all of that money, then roll the dice.