A restaurant could cut the portion in half and reduce the price by more than $2, using your numbers, IF doing so would increase the number of customers that order appetizers and dessert.
I don't think you can really stretch more than $2 out of the deal. Appetizers at Prime Time Cafe, for instance, run about $6, and dessert is about $4.50. So already, we have a hard limit at $10.50. That's assuming that those prices are
all profit (they're not), and that
every patron ordering the half portion orders both appetizer
and dessert (doubtful), and that none of those folks are sharing entree, appetizer or dessert (definitely not). Every equivocation there eats away at the amount that the restaurant can afford to cut the price of an entree and end up revenue-neutral. In the end, after all the equivocations are factored-in, I'm pretty sure that absolute best isn't much more than $2.
The restaurants could also generate revenue by charging a plate or sharing charge.
This is viable, financially, but customers generally react negatively to the idea of a plate sharing charge. We should post a poll about that.
Assume your numbers and a $10 sharing charge.
I think you'd have dozens of DIS members yelling bloody murder at a $10 sharing charge. I think the best you can hope they'd accept is $5, and that's just not enough.
Large portion sizes give customers the appearance of value and reduces the pressure on the restaurant to provide quality.
Absolutely.