OK, I know that your OP asked the question about how much more would it cost to increase sides, and I have no idea about that. I have, however, thought about this and after reading the entire thread and all the responses of those here that run restaurants or cater (or both) I'd say that I see their side of it.
It wouldn't seem like increasing side portions would cost all that much, but as I understand it profit margins are a very important and critical thing. AND easy to eat away at.
Like Vinny and lionhrt said, a simple addition of anything, no matter how seemingly insignificant adds up and comes right off the bottom line.
AND, as Bigmista said, the thing that brings people into a good BBQ place is the BBQ, not the sides. I go for the meat. I always order fries and slaw because I prefer those 2, but I only really need/want a taste of it. I go there for the ribs. If I wanted slaw or beans, I can get that down the road at the grocery store.
Ask yourself this question.......would your decision between two BBQ places be made only on sides or would it be which one produces a better product of meat? I mean, if one place gave you a chitload of sides, but had not as good of Q, would you chose that place? I'm guessing most of us would say no. As Mista said....sides are really incidental.
I get your point buzzard, I do. But I think the answer to you're general question of how much does it really cost is......it costs too much to do it. Whatever the number is, it's too much. If not, everyone would do it, but they don't.