It's a "gamism." It's true that you can get a set of throwing knives for about $20-$30 on amazon, and that combat knives (not thrown) cost anywhere between $20 - $100, possibly more depending on quality. I couldn't find any good examples of shuriken that weren't toys.
If the break from reality is too much, you should talk to your GM about it, perhaps you can come to a work around on getting cheaper throwing knives that are one-use only, or something along those lines. It would have to be a house rule, though.