As I've said before, I've run pretty much everything from +3.5° caster all the way up to +8° caster on my Duster. I do my own alignments, so I've arrived at running +6.5° by running both more and less than that.
When I went above +7° the steering effort began increasing dramatically, but high speed stability didn't seem to improve dramatically with it. When I went below +6° I started noticing some "tracking" coming back into the straight line stability for the steering and dropping lower than about +5.5° didn't seem to continue to make the steering effort noticeably less, but it increased tracking further. So the +6.5° I run now is basically the "sweet spot" for balancing stability and steering effort, at least on my car running 275/35/18's up front. The tracking issue has a lot to do with how wide those tires are, the wider the front tire the more the tendency for them to track the road imperfections. So with a 245 up front you might not want/need +6.5° to get your straight line stability.
As for gas mileage, I'm not sure you're thinking about this correctly. Driving straight ahead caster has no impact on the contact patch, and driving straight ahead is when you're going to have the most tire area on the ground and therefore the highest rolling resistance or what you called drag.
As you add caster to the alignment, the wheels will tip more on edge as you turn (more camber). That's where the added steering effort comes from, tipping the wheels more on edge is basically trying to lift the car. So steering effort increases.
But drag? Nope. Tipping the wheels on edge is actually going to decrease your contact patch. Less tire area on the ground, less rolling resistance. Now, if you've matched the camber gain from your suspension and what you're getting from the caster angle to the body lean angle of the car in a corner, you're going to keep the contact patch on the inside the same. That's the goal, maintaining the tire contact patch on the inside wheel. If you haven't done that and you've got too much (or too little) camber on the wheels you're actually reducing the tire contact patch while you're cornering. So, less tire area on the ground, less rolling resistance.
And sure, too much camber will wear out the tire on one edge because of how the tire is angled. But that's not because you've added rolling resistance, it's because a smaller part of the tire is bearing the weight of the car. Less tire area on the ground means higher pressure on the tire because it's bearing the same amount of weight. But the friction coefficient of the tire doesn't change, so, less area on the ground means less friction.
