Just to clarify: The Thunderbolt came with a promotion that allowed free Hotspot use. However, that promotion has ended, which was disabled with a recent OTA update. If you haven't received that update, you should still be able to use the Mobile Hotpost feature promotion free of charge. When you turn on the Mobile Hotspot app, it will tell you either that it's free to use, etc. Or, will give you a message to subscribe to the mobile hostpot feature. On mine, I had to have it added manually, the self-provisioning through the phone wouldn't work.
But, you do pay the extra for that, $30 a month if you were on the plan before VZW switched to Tiered pricing, or an extra $20 if it was after that. The first thing, would be to check the Hotspot feature, see if it's updated to require the subscription, or if you're still covered under the free pre-OTA promo.
Note, that applies to stock, non-rooted phones, as Rooted users have other options as well.