T-mobile has a $30 prepaid plan that gives 5GB, 100 minutes, and unlimited texts. You can only get it by signing up online. It tethers for free as long as you don't update your phone to the newly released T-mobile carrier profile which now forces you to officially pay for tethering. But even if you had to pay for it, it's just $15 extra so your total would be $45 per month and you can tether up to 5GB. It doesn't give you many minutes to talk, but if you go over, it's just $0.10 per minute, so might still be the most economical plan. It would rely on you having an unlocked device already that you could use with the plan.