At some locations, upgrades at the desk are kind of a combination of the agent's whim, directions from the manager on duty, and a few other factors (average rate, number available, etc.). There's not really a hard and fast rule.
At others (and--I believe--most corporate locations, of which MCO is one), there is an upgrade table that lists the standard upgrade prices. I'm not sure if it's actually used (I heard something once about it Blue Chip customers getting upgrades at 50% off from that table, but I'm not sure if it's used for normal retail renters). I don't know what the number on that sheet is, and I suspect it changes every so often, anyway.
I think I found and posted the phone number directly to the MCO office recently. You might try searching recent posts in this forum by me to see if you can find it. If not, post again in a day or so and I'll look it up. A call directly to them should be able to clarify their procedure on this. (I'm not in a place I can take the time to find it now, but I may be later.)