Hi Gaz,
Yes, those ticket prices sound about right.
Try
http://www.broadwaybox.com for discounted tickets We got 2 tickets for Curtains for only £78 from there. It's too early for June now as they usually only have discounts 8 weeks in advance but you could take a look now and it'll give you an idea of how much you can save.
Wicked is incredibly popular and if she really wants to see that then I'd order your tickets before you go and you may have to pay full price for them as often the popular ones aren't discounted. It's doubtful you'd get any for Wicked from the booths yeltzman mentions; as he says it's often pot luck as to what they have, where the seats are and you'll be queuing for ages (there's far more exciting things to do in NY than queue!)
Hope that helps.