You are charged for a month, or pro-rated if you close out the feature early. So, the $30 a month plan is for the unlimited tethering option for those who are grandfathered into the unlimited plan from back before it changed. So, $1 a day. And it doesn't matter how much you use in that day. It's effectively a dollar a day, for however many days you have it on for the month. If you only have it active a day, you'll only see a day's worth of charges.
If you're on the new data plan, where it'll be a capped hotspot feature (i.e. the $50 data plan - $30 for your 2GB phone data, $20 for the extra 2GB for hotspot), you're still in the same deal. Only instead of being charged $1 a day, it's more like 67 cents (20 a month div by 30ish days per month). However, it'll show up differently on your bill, since you're dropping the $30 data package for phone only, adding a $50 plan for data + hotspot, and then going back after you switch back. The end result should still result in basically a "per day" difference.