With most major carriers, using your phone as a hotspot doesn't cost any extra because the data comes out of a prepaid plan’s data package at no additional cost. Most plans cap the amount of general data allotted to hotspot usage. When this amount runs out, you’ll no longer have a hotspot option, or the connection speed will be significantly slower. In this case, if you still want a hotspot, you can buy additional hotspot data or wait for the next cycle when your plan's data renews.