For some reason, the flights to LAS have really decent YUP fares (what I think you may be seeing as 'First Class' fare). Perhaps the reason is that so much of the fares are low low cost fares that when they fill up the higher fares become much easier to book.

The YUP fares to west coast cities (for me from MSP) are always pretty much the same .. same $$ to quite a few cities. These fares are normally more than twice the YUP fares to LAS....

As EXP I do not look at the issue of sticker 'cost' for upgrade . but the convenience and comfort of upgrade clearance. While 100 hours is a nice time frame, many cities (LAS and MIA for example) tend to be very full in First anyhow (either YUP or in the MIA case international connectors in Business) so my upgrades tend to clear later or at the gate.

My trip to LAS last year I booked the only available discount ticket .. which I did not think was too cheap. A couple weeks later as I saw seats in First get sold and the upgrade looking a bit shaky, I discovered the YUP fare. I called AA and yes could pay the difference from my current fare to YUP (about $150 as I recall) but they wanted to hit me with $100 change fee. I balked and the agent eventually got approval and waived the change fee. I think at the time coach was pretty well booked so it freed up a seat AA could sell again. Win Win situation for them.

NOTE that the same concept (discounted tho restricted upgraded fares) is really nice provided you do not need to change your itinerary. I use ExpertFlyer to look at all the fares and see which is best and gives me an upgrade. I found the 'I' fare to Europe (discounted restricted Business class) to be a decent option compared to the eVIP wait list circus.