I have a smart phone but can not afford the cost of a monthly plan.
You do realize that you don't need to have internet access to use the authenticator, don't you?
Once you download and activate it (it can be done in any wireless network) you just have to use it offline.
This. I use the authenticator on my iPod Touch. I also have it on an old smartphone for which I no longer have a SIM chip, in case I drop my iPod in the toilet or something similarly stupid. There are even authenticator options that run on your PC - an open source one called WinAuth (If you are willing to trust a third party with your b.net security) or an emulated Android device to run the Android authenticator. While these are obviously less secure than using an actual separate device, they are still better than not having an authenticator at all.
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This. I use the authenticator on my iPod Touch. I also have it on an old smartphone for which I no longer have a SIM chip, in case I drop my iPod in the toilet or something similarly stupid. There are even authenticator options that run on your PC - an open source one called WinAuth (If you are willing to trust a third party with your b.net security) or an emulated Android device to run the Android authenticator. While these are obviously less secure than using an actual separate device, they are still better than not having an authenticator at all.