Theater chains often use the debut of a big new movie to slide their ticket prices up a notch, so last week’s $12.50 ticket price has been tossed in the dustbin of history. “Iron Man 2” fans, you’ll be paying 13 bucks to see it in Manhattan — or $19.50 (gulp!) for an IMAX screening. And it’s not even in 3-D. (Kids and seniors get a break — they’re only out 16 clams). I see that tonight’s midnight screenings are sold out, but get in line now for those 3 a.m. showings! Tickets for movies being shown in 3-D are still being priced at $17.50. Not for long, I’ll wager.