So, based on those, to me, a good amount of fries at a restaurant for $5.79 would be around 4 pounds.
Most restaurants typically aim for around 30-35% food cost when pricing their stuff, with the rest of the cost made up by the labour and other expenses. You're suggesting 50%, as well as basing that suggestion off of the purchase weight of the potatoes. For fries, purchase weight and actual weight are pretty similar, since you're just cutting the potatoes up and frying them (unless you peel them), but you will still lose some weight as water evaporates during the cooking, plus the inevitable bad potatoes that can't be used, all of which means you're suggesting even higher than 50%. That's not really reasonable pricing for such a common item as french fries.
Not to mention most burger places make their money by selling fries+drinks, the burgers are the least profitable items for them and while they don't lose money selling those they make the lowest margins. It's why they always try to get you to buy the combo meal in a lot of places.