I don't know about slipping a gate/counter agent a bill. I don't think it's going to work. Airlines have strict policies about upgrading their prepaid flyers and their elites. Also they have only a set amount of seats on a flight. It's totally unlike a hotel where there can be plenty of rooms.
Have you tried getting tixs thru reputable consolidators?
There are plenty of hotels of all grades and costs around SFO, why not Priceline one?