Yes, buying the packages ahead of time always saves you some money, but how much can vary quite a lot. This close to your sail date, you'll have to pull the trigger quickly if you want to do it, so you'd have to accept whatever whatever the current sale is--which might be a really good one, or might not. ๐ We always buy a package so we book it when the sales are best.
They do offer specials onboard as well, so if you keep a close watch on the daily Cruise Compass, you might find something that would interest you.
Bottom line: yes, you will pay more onboard, but if you're not sure you actually want them, that's a pretty significant amount of money to pay just in case.