It really depends on what the studio expects, I'd say, and what their long-term plans are for a given property. The numbers for Batman Begins, for example, were solid, but not overwhelming ($205m US on a $150m budget), yet it was an improvement over the last entry in the franchise, Batman and Robin, and so it proved that Nolan's reboot had worked. They were apparently going for a bigger sequel, which they did, and which, as we all know, paid off big time. So if Paramount is looking towards a trilogy or so of films (and it seems that they do considering they already placed a script order for a sequel), a number for STXI they would be satisfied with might be somewhat lower than what one would expect. Maybe they would be satisfied with the film breaking even domestically, banking on overseas as well as DVD and merchandising sales to bring in the profit, if it means that this film reestablishes the franchise and paves the way for more profitable ventures in the future.
Or not.
But then again, I doubt they expect mega-blockbuster numbers. People in film studios are notoriously cautious when it comes to spending money. They want safe (or should we say likely to be profitable) investments. Since the franchise has been in obviously bad shape the last ten years, they must have some security somewhere. Otherwise, I doubt they would spend $150m on a property that hasn't done so well in the past. Maybe some of the licensing deals related to Star Trek were very lucrative for Paramount so that a good part of the investment has already been made back before a single ticket has been sold?
Also, let's remember Nemesis. A box-office bomb by all accounts. And still, they didn't let Star Trek die. So obviously, the entire industry behind it, the merchandising, the DVDs etc, saved it enough to warrant another attempt. Star Trek may be one of the safer bets in the movie business because of that...