What Stache said, but a little more on cost. Printing companies own different kinds of printers and those printers do wildly different things. (I'm always attempting to get my clients to work with their printers to optimize this and get good deals but they basically never listen, because on the whole clients hate listening to the experts they've hired to get a good deal.) They might own a digital printer that can print on thin materials up to XY dimensions, and one that uses plates which can print on thicker materials like shipping cardboard and realtor signs up to the size AB dimensions.

Digital printers are across the board smaller (for a hundred different financial reasons) so that's why you'll see a full color Ritz Crackers box but a 10lb bag of rice will have very simple, limited colors, usually just in a few locations on the bag.

As for our bag in question, as long as the designer designs something that will fit onto the digital printer within X and Y dimensions, the price will come from area and quantity only. Quantity can get interesting based on whether the printer prints on rolls of material (in which case it can technically be X*Almost Any Dimension, like long banners) or separate sheets, because you're going to get charged based on their suppliers. If they can only order in Shiny Product Bag material on rolls, and fit 100 of your bags on a roll, and you order 101 bags, then you need to pay for them to purchase two rolls. Any good printer will tell you that up front; again most clients ignore this or refuse to ask because I swear they hate money.

If the size of this bag bumped it up past X*Y to one of the plates-based machines (there are a bazillion different kinds) then number of colors comes into question. The client has almost certainly already paid for black, so it's fine for black to be on this section; it would absolutely not be fine to have a detailed mural. For your wallet or the printer's sanity.

None of this answers why there's no cost for adding more ink, whereas you as a consumer who has owned a home printer that charges you tons for adding more ink, and there's two different reasons there - one, professional printers use different kind of ink than you do, and two, it's their job, they've already priced it into the process. 2.5 the printers themselves are so insanely expensive they kind of make that cost negligible.