Depends what you're adding it on to, full-board? I did the math for me and my family, I think it was like $500/night more for all-inclusive versus a regular rate which had breakfast only, the all-inclusive rates also include a massage for up to two adults plus a dolphin tour or fishing trip for everyone in the room. If you book through Prive you also can get a $100/night credit towards food/drink and spa, that may be an option if you don't plan on drinking much.

One thing I don't think I mentioned before that I wasn't sure about when going, all-inclusive also includes the drinks in the in-room minibar except the bottle of bubbles (it does not include the food there).

I preferred not having to worry about how much we were spending on every little thing, but I also had kids who like to have a Fanta at every turn that would have really added up! I also drink my fair share, my wife less so.