Other Car Rental Programs (ie. Alamo, Enterprise) - Upgrading to a Premium from a Full-size w/ Alamo

So here's the deal -- I'm going to need to rent a car at LAS in early November. I'm interested in getting a premium car, but I also want to be a cheapskate. :)

Now, I'm not sure if I'm going to be especially smooth as to earn a free upgrade (as described by http://www.flyertalk.com/forum/other-car-rental-programs-partners-ie-alamo-enterprise-sixt/1242177-alamo-upgrade.html) -- but I was wondering about how much upgrades tend go for if I need to pay for them?

Basically I can do a 4.5 day rental on a full sized car for about $206 ($113 pretax). It seems that a booking a premium goes for $280 ($190 pretax). Am I just better off booking the full size and trying to snag an upgrade later, or should I just bite the bullet here?

(Rates here use the costco rate code + a free day for the full sized car, and costco rate code + $30off for the premium)

If they're lightly loaded and you're smiling and they're smiling, I've found you're odds at free are OK but not guaranteed; but about a million times more likely then with say Discount or Budget.

If you have to pay, it depends on how well you can haggle; Last I tried the pricing started at $25/day and fell the longer I negotiated but I don't know how low they'll go.