To set the record straight, your smartphone's GPS is free. It is basically like using Bluetooth; there is no additional charge from your carrier. What you do need to watch for, however, are programs that use mobile data in conjunction with GPS, like Google maps ATT's stock Navigation app. These programs use data to load the maps they display, incurring fees from your carrier. There are two ways around this. The first is to use wifi instead of your data plan, bit this is not practical if you need to be mobile. The second is to use an app that stores maps on your phone, eliminating the need to access the internet. Tomtom has a paid app available, but I prefer free app Locus, which allows you to download maps of your choice to your phone to be used later.
If you call your mobile provider to ask about this, be prepared for some confusion. ATT informed me that all apps use data, and offered to sell me a more expensive data plan. Be advised: if you have the mobile data setting turned off on your phone (I have a widget just for this purpose) then you cannot be charged for data usage. Apps requiring it will simply not work. If you use ATT, you can check your data usage by sending a message to *DATA# (*3283#) to check usage.