I don't play the shinobue but I do play keyless flutes (mostly in the Irish style). $100 is really pretty reasonable for a bamboo flute, but you can still get decent handcrafted bamboo flutes down to maybe $50.
When you see one for $15, it was probably mass-produced, and you simply cannot mass-produce bamboo or wood wind instruments. They may -look- the same but they won't -sound- the same. They may not work at all, or be horribly out of tune, or just have an abnormal sound. If you're at a shop and can try them, you can get lucky, but I'd be concerned about buying at that price without being able to test it.
(Of course, you -can- mass produce perfectly decent instruments at that price - in plastic or metal, which are very easily handled by machines, if you decided to play the recorder or the pennywhistle instead.)
I'd be careful and not jump at anything that looks too good to be true. Check that you can return for a replacement or full refund any instrument that doesn't work. (You may need someone who can play the flute to check - some people take to it immediately and some take days just to get a clear sound. Fortunately, all side blown flutes play pretty much the same so any kind of flautist can check the sound, they don't have to know the shinobue.)