Cost depends on where you are, and what kind of plan you have. Up here with Telus, it's free to use the "Personal Hotspot" feature on my iPhone to connect a number of devices, and I've got a 5GB data plan, so I'm pretty good.
So why did I order a 4G iPad? First of all, the iPad uses LTE, and it should be a considerably faster connection than my iPhone has. Second, the LTE models have a "proper" GPS chip in them. Third, using my iPhone as a personal hotspot eats the battery like a fat kid eating candy. Oh, and fourth, I can set up a "flex plan" for my iPad that ranges from $5 to $35 per month, depending on how much data I use that month. The cell company's are finally cluing in a bit, even though I'd prefer they adjusted their tiers a bit with regards to the cost bump-ups.
C