What are the typical additional monthly costs for having cell service?
I realize I'm in the minority these days not having a need for a cell phone. Coverage in my area is poor and unreliable.
What kind of accuracy and reliability can one expect from cell phone service in remote locations regarding GPS use?
My friend from the EU is in New York for a couple weeks and we setup a T-mobile plan a few years ago for when she visits. Gave her my old iPhone 4 - even though this is about a 5 year old device, I would bet it has 2x or 3x the cpu of current Garmin handhelds as well as 32gb internal memory. You should be able to get something like this very inexpensively (although newer is better, perhaps an iPhone 5s).
Anyway, when we first opened her T-Mobile acct, it was necessary to deposit a minimum of $10 every 3 months to "keep it alive". This would be their "pay as you go" plan where you only pay for the calls you make. The money just accumulates in the plan if you don't make any calls. After a year (IIRC) they said she was a "loyal customer" and now I just deposit $10 in the account once per year and it remains active for the next time she will visit the US.
Just looked at their plans and they are very confusing, but from what I can tell, $60/ month gives you unlimited US calls (maybe canada too?) and unlimited text messages. It also includes (I think) 2gb of data, but that is on their highest speed network. I think it is actually unlimited data, but after 2gb it will be slower. This is what she has while visiting, but she... talks and texts a lot.

IF you don't really need that much, you can just use the pay as you go rates, and keep adding money to your account as needed. No idea how good their coverage is.
I have used both Verizon and AT&T and the coverage is very similar, and widely available except for remote areas. Cell service is weak where I live, can't usually get a decent signal inside the house. Out in the yard it usually works with a 2-bar signal. I have a device called a Femtocell at home. It's a miniature cell tower that just covers the area around the house and sends the call over my DSL connection. It's all user-transparent, works just like any other phone call. All the companies have some version of this, mine is an "AT&T MicroCell".
GPS chip quality can vary, the newer models are generally better and include GLONASS. If you want better accuracy, you can use a bluetooth device like the Garmin GLO. I have reviewed this at GPSTracklog several years ago, and also did some testing with it recently - see:
http://forums.gpsreview.net/discussion/30108/what-is-gps-accuracy-testing-the-garmin-glo-in-the-forestFrom your question, not sure if you understand. Modern smartphones have standalone GPS receivers inside, they do not require any cellular service to operate. If cellular and/or wifi signals are present however, they can use these to augment the precision of the GPS fix.
At the operating system level, iOS chooses whatever source is most accurate. If you read my report about the Garmin StreetPilot app in New York City, I found it better at showing my position than a dedicated gps. I assume this was due to cell and wifi signals. It even tracked me through the Lincoln tunnel as I sped up and slowed down with traffic.
