Hi Elisa, the Nike website won't say this (of course), but you do not need to purchase the Nike shoes to use this. I got a little pouch (
www.marware.com) that holds the chip on the top of my laces, and it's worked without any issues. If you are a crafty kind of person, you could probably sew your own version too - but I'm about as clumsy as it comes at that kind of thing, so the marware holder was worth it to me.
Also, the price for the kit is not that much - only $30. You have to have an ipod nano to use it, but if you already do, then the cost of the kit is pretty reasonable.