There are different ways you can approach this.
IF one of your main goals is to make the trip as inexpensive as possible, but ensuring you get certain “must-dos” including a Character Meal, then I’d wager you could save more money by not having the dining plan and being frugal with how you eat, etc.
If you’d rather not have to worry about what to eat based on cost, then there is value in that, as well, which then means that even if you aren’t necessarily saving money on the dining plan, it still is worth something more than money.
In my own calculations, we couldn’t make using the dining plan worth it for us. But we actually went through each menu and picked out the items we were most likely to eat, plus added in snacks and drinks, to see where we fell…and then did the same on the dining plan. The dining plan always ended up costing more. (BTW, no need to add in gratuity when doing the calculation because the cost of the gratuity would be roughly the same in either scenario, so it becomes a wash.)
Now, if you want to pay roughly the same price as you would by NOT being on the dining plan, but then be able to splurge on a few dining options where you otherwise wouldn’t to “save money”, then again, that should be weighed.