We only tried the DDP once, last December (free).
I ordered some snacks for the room and wine from Garden Grocer, as we were going to try ME for the first (and last) time. ME took over 2 1/2 hours to get going to our resort! Never again. And, I didn't need the wine, or more than one bottle, as we weren't in the room much. And the snacks (breakfast food) was overkill!
I had no idea how much food was on the DDP! My DH wasn't joining us until after the first couple of days, so we had a few extra credits to use. First night, the kids and I had room service (2 TS credits, we were at the BWI and honestly the food sucked but the service was outstanding, so it was all fine, we were so tired). For ONE snack credit, you could get a bowl of cereal plus milk at the Boardwalk Bakery. Plus, they have a CS credit option: a sandwich that is HUGE, plus a side and a drink.
The only thing is, I do like to drink, so I do pay for that. Plus, yes tips.
We ate so well. I would definitely say it's worth it if you are paying for it, as it is nice that things are already paid for. Plus, my kids thought it was fun, to get little snacks all the time! We enjoyed "spending" my DH's extra credits before he got there. We probably left with one CS credit unused, which was fine.
So, I'd factor in just alcohol and tips. Or maybe a couple of extras, but not much.