Two cihoices: get a data plan for the iPad, starting at 20 a month up to more than 40 depending on consumption. Better yet: tether the iPad iPhone and use the data on that plan. Provided your devices are always with you, thatnworka well.
I have that plan too but I've never tethered devices before. Does it just use your 6 GB from your phone? Does your phone still work while you're tethering?