I've noticed it seems to really vary. With my old Verizon setup, it would have gone from $70 a month for 700 min/500texts(unlimited to other VZW)/Unlimited data, to $100-$120 for Unlimited text/minutes (I less than 200 of both of these a month) and 4 gigs of data.
Well, you must be grandfathered into an older plan then. Verizon no longer offers any unlimited data for the iPhone. So for someone currently on AT&T, that's not an option. I looked at Verizon, and they had a similar plan as the mobile share for the same price as we're paying. AT&T offers better coverage where I'm at so I stuck with them.
For a user on an AT&T family plan though (like most of us are/were), buying individual data packages, the mobile share can be the same price or less, but have the advantage of sharing data AND tethering. My wife doesn't use anywhere near 2 gigs, but I do. AT&T previously offered 250MB or 2GB. Wife uses more than 250MB! With the mobile share, I could do the same 4GB of data as before, for the exact same price. But, sharing them across two devices means she can use her 500-600MB she always uses, and I can safely go over 2GB because I can spill over into what she doesn't use. (Though I did end up getting the 6GB plan because it was only a few bucks more, and I've been tethering a lot lately)
For two iPhones and an iPad, unlimited voice and text, 4GB data shared is $160/month
For two iPhones and an iPad, unlimited voice and text, 6GB data shared is $170/month
It's 10 bucks more you get 6 gigs because, when you go up to 6GB, you get a break in the per-device rate (So you pay $20 more for 6GB of data than 4GB, BUT, pay $5 LESS per smartphone to have on the plan, so it comes out to $10 more for 2 smartphones)