Hard to say exactly, but down a bit is a good bet.
First, Beckham's last 9 games were downright absurd, and likely not repeatable. 1199 yards and 9 TDs across 9 games, is crazy; there's little reason to think he'll be able to do anything like that across 16 games, which would put him north of 2100 yards. Seeing as no one has cracked 2K yet, somehow I'm doubtful.
Now, the second question on that though is whether his per game will lower but he'll still achieve 1300 and 12 TDs. This, on his own, is more reasonable, and doable, a smoothed out fantasy point total is not unreasonable.
But, as you asked about, Cruz does add a wrinkle.
It's always tough to know exactly how an offense will perform with players in it until we see the unit working together on the field.
That being said, we can look to the past few years of Giants teams to get a little indication, specifically to the 2011-2013 time frame, starting with Cruz's ascension, and the aftermath involving Nicks and Randle.
In 2011, Cruz hit the scene and had a crazy season, over 1,500 yards with 9 TDs. The next year, 2012, he fell back towards Earth, still cracking 1,000, and actually scoring 1 more TD with 10, but it was a worse season overall.
Then, in 2013, you had a full 3 WR corps, with Cruz, Nicks, and Randle. Cruz's numbers continued to drop, sitting at just below 1K with only 4 TDs.
Beckham may follow the same trajectory, and his 2015 is probably likely to look like Cruz's 2012 or 2013 over repeating Beckham's 2014.
With Cruz back in the mix, you've got a WR corps reminiscent of 2013, with 3 very competent WRs vying for the ball, plus if Larry Donnell maintains, he could be soaking up targets as well.
That limits Beckham's cap. I do think he'll remain at the forefront of that receiving corps, but I am thinking his totals drop to maybe 1,000-1,100, with a TD total in the high single digits.