Oh, for the halcyon days of my youth, long before Scott invented the cell phone while munching on a deep-dish pizza, back when the better part of my days were gloriously free of my mother's nagging.
Sigh.
As for your question, I would consider a prepaid service. It's usually much cheaper (although I don't know how it would compare to a family plan). But there are caveats.
The first thing you have to consider is data caps. With the exception of T-Mobile (whose unlimited data plan really is unlimited -- they don't even care if you tether your PC to your phone), all of the other "unlimited" plans do have caps. They just craftily work them into the TOS as prohibitions against using the data service for, well, pretty much anything other than basic Web browsing and email, which allows them to terminate you if you get carried away with it.
I've also received anecdotal reports that Boost Mobile is pretty liberal with data. My goddaughter watches a movie or two a day using Netflix over Boost Mobile, and they haven't cut her off. So maybe Boost's "unlimited" plan really is unlimited, too.
The other thing you have to consider if you go prepaid is what service will actually be carrying the signal. Most prepaid providers are MVNOs that lease time from major wireless providers. If one or another provider's service works best where you live, then you would need to find an MVNO that uses that host.
Sprint offers the lowest prices to MVNOs, and is therefore the most popular host for low-priced MVNOs. If Sprint is good where you are, then you can save a bundle using Boost Mobile, Virgin, or any of the zillion other MVNOs that work over Sprint.
If Verizon is your carrier of choice, then I can tell you from personal experience that the absolute cheapest way to get VZW is to go to Wal-Mart and buy a Straight Talk phone that has "CDMA-V" printed on the box somewhere. If you want Sprint, look for CDMA-S. If you want AT&T or T-Mo... well, more about that later.
Verizon's the only game in town where I live, but I get my VZW from Wal-Mart through Straight Talk. Why? Because I'd rather pay $45.00 a month than $120.00 a month for unlimited everything, and the service is almost identical. The only thing (other than possible data caps, which I have never personally run into) that makes the service "almost" identical is that my VZW service doesn't roam.
So that means that if there's no VZW signal, then I have no service. This is fine with me, though, because I actually enjoy being out of cell phone range. It reminds me of the days before the blasted things were invented, which makes me all warm and fuzzy inside.
If you want prepaid service that does roam, then consider T-Mobile. Voice and text (but not data) will roam to AT&T. You'll also get T-Mo's truly unlimited data plan when you're on their towers, plus the ability to use literally any T-Mo phone (rather than the rather crappy selection that MVNOs in general provide) with your service. If I had a T-Mo signal here, I'd go back to them. Just having the tethering as an available backup for my cable Internet would make it worthwhile.
I'm also told that if you purchase a Straight Talk GSM phone and physically activate it in a place where AT&T has a signal but T-Mo has none, then the phone will be assigned to AT&T as the primary carrier, with talk and text (but not data) roaming on T-Mo.
If you want T-Mo on Straight Talk, then just activate a GSM Straight Talk phone someplace where there's a strong T-Mo signal. If the phone finds strong T-Mo when it's activated, then it will default to T-Mo's network.
That's what I'm told, anyway. YMMV.
Here's a list of some of the Host / MVNO relationships, offered with no warranty regarding it's accuracy:
http://en.wikipedia.org/wiki/List_of_United_States_mobile_virtual_network_operators
I have a friend who's in the prepaid cell phone business, and he knows pretty much everything there is to know about it. If you decide to go that way, let me know, and I'll send you his information.
-Rich
