Since when is VZN "$50 for 2 phones"? Even if you got 2 Blackberries and sub to VZN Global on both, thats $40/mo. VZN is $10/mo, and if you have a regular phone (not a BB or WinMo) there's a $3/day option you can use if you only need GPS very occasionally. And if you do have a smartphone with a non-locked GPS and already pay for a data plan, you can use free options like Google maps or MS Live. Even if built-in GPS is locked to work only with VZN (their standard trick), a $40-50 bluetooth GPS gadget will work very well.
Of course, main pitfall of phone-based navigation is that it dies when you are outside primary coverage areas, and now and then data reception is kinda finicky - though for the most part its quite good on my VZ Moto Q9C .
But frankly, it's kinda tough to beat the convenience of a dedicated GPS unit. We got a Garmin nuvi 765T in our main car, I love it - not cheap though. Map updates... nah. Roads, hospitals, gas stations and such for the most part tend to stay put - friend of mine has a never-updated nuvi that's 3+ years old, it's still spot-on. If you are in an area that recession hit hard, lots of businesses in the POI db might be gone. Since POI db usually lists phone numbers, might be a good idea to give them a call before driving there.