Would I pay $1 to go from a Focus to a Corolla? Good lord no! But I'd still go for the C reservation.

In fact I'd pay to go the other way. The problem is that most B cars are not like the Focus - they are smaller. The Fiesta is in the same group and a Nissan Versa is even a group C. I'd therefore always be in at least a C to have a shot at whatever is in Gold Choice.

If however you would be just as happy with a Yaris as with a Camry, keeping the group B car would be fine.

As for the rate - generally pay now with Amex is as low as it ever gets. If you put in a PC then it tends to make the rate flexible, but a good PC tends to keep the rate about the same (or very little more). Personally I value the flexibility so tend to use flexible rates, or switch to prepaid at the last minute if I know that I'm definitely taking the trip.

The only way I can see you improving on $20/day is to rent from an off-airport location.