G.I. Joe had a less than 60% drop. Anything below 60% is generally considered "not bad" for a major summer release. It's 60+ percent that indicates "poor word of mouth." It's also worth noting that Joe performed quite well on Saturday, and did reasonably well through the week, which tends to indicate that it's doing pretty decently in the "kids" market.
58% at this estimate to be exact which is dancing on 60%, but not sure where you got the numbers for "not bad" as its not quite so cut and dry as you are explaining (much depends on the open which in joe's case was not bad, but also not great).
This movie is already out of "disaster" territory unless everyone on Earth stopped going to see it right this second, and plainly that's not going to happen. "Disaster" would have been a $40 million or less opening weekend. It isn't going to be a runaway hit, and it's certainly not going to make $300 or even $200 million domestic box office, but it's performing reasonably well internationally, and will likely make up any "gap" on DVD and with residuals from merchandising and the like.
I would definatly not say its not out of disaster territory, it still has $75 million to make budget domesticly BEFORE you count in marketing costs (another 150 mil).
Internationally its really not doing well, last weekend The Ugly truth beat it in England, I mean that is the Kathrine Hiegl and Gerard Butler movie that has been out for a while and still out performed joe. England is a good gauge for the rest of Europe as well (which is where a movie really wants to hit a high note internationally).
On international numbers look back just a few years ago at Golden Compass, Domesticly it barely made budget but internationally it made huge numbers, but a sequeal (that had been set up) is not going to be made due to the Domestic responce. Similar situation with Narnia: Prince Caspian last year as well and same outcome.
Also the residuals from merchandising don't cout to the movie's totals, Movie companies just consider it "icing on the cake"
I'm not trying to sound all doom and gloom but reality has to enter into this. Truth is paramount can do what they want and even if the movie hadn't made as much as it did, they could still do a sequeal if they wanted, money is thiers to invest as they see fit.