In case you are wondering, the reason why bluetooth is so expensive is because it supports much higher data rate (this is irrelevant for your project), has high resistance to interference, and supports encryption/authentication.
In your case, only the interference resistance may matter, especially if you want to do multiple connections in close proximity (you can't do that with the $9 pair, because they don't do any frequency hopping, and doesn't support channels).
But then it only costs a tiny fraction of bluetooth.