Same here. Last year heading to my sisters wedding, I'd been allocated a VW Passat. Looking around the lot, there were plenty of more desirable cars such as Audis, Mercedes, but when I went and asked if I could pay to get an upgrade, they would only reprice to the 'walk up' rate for those cars which needless to say is very expensive.
The times that I have landed good upgrades have been when I've booked a model that was then unavailable at time of pickup. For example I had a great run of booking 'green collection' intermediates, and getting upgraded to prestige or fun collection cars. In the last few months I've not seen a single upgrade though, and I can see why, because I'm seeing far fewer high-end cars on the lot. I guess they've scaled back because of the economy, so there is no oversupply anymore.