Surely the RTTI on Tom Tom uses data as well? It hooks to your phone.
The newer ones with 'Live' services (traffic, speed cameras, weather, places) use their own internal sim and aren't linked to your phone at all. You also get free lifetime maps (4 per year) - maps and speed cameras were both subscription services on my old one.
I paid £239 for the new one and that's it for life. No ongoing costs at all