It is, but a lifestyle isn't equal to what you earn. Plus, since at the moment lifestyles have a minimal in game effect, all this would do would encourage people to take low lifestyles except for the diehard RPers. So even saying "tax rate based on Lifetyle*2 or Lifetyle*3 doesn't work.
the taxman will get really suspicious if you're paying $2,000 a month for rent, and claiming that's all you make in a month, after all.
It will probably just be a flat "yearly" rate, payable by the end of Month 12 of each the Missions calender. I need to talk with the rest of the writers and stuff though and see what we can figure out.