I'm a really big fan of the Deering Goodtime 2 as a beginner banjo, though in looking for prices, it seems I either got a really good deal or the price has gone up recently. I recall paying around $350 for a new one from a great music store in Staten Island called Mandolin Brothers. Now a days, they look to be running closer to $500.
You can certainly find banjos for cheaper. The major music stores seem to carry fender models, though I really don't enjoy the feel or sound of them myself. It really depends on whether or not he intends to keep playing or if its a passing phase as to whether or not you are willing to spend more or less.
I started on my Goodtime2 almost 6 years ago, and have since "upgraded" so to speak 4 more times, with banjo #5 being built as we speak. But the remarkable thing is that in spite of having a selection of banjos to play, I'll still play my Goodtime just as much as the one several times its cost.... its got a great sound, and its light weight makes it easy to fly out to festivals with, so I guess thats my recommendation.
In a more general sense, however, you can probably expect to pay between $300-$600 for a beginner banjo that will play/sound well, so if you see something for much less than that, I would be hesitant to purchase without having someone play it for you first.
Those are my 2 cents
Adam