Positive it was 6? I know at Discount/America's Tire, MOST vehicles it's 3.50 a wheel. For MOST stem mounted sensors, they replace the rubber grommet, retaining nut, valve core, and if applicable, a metal washer.
Some Fords use banded sensors, meaning the sensor is mounted to a metal band wrapped around the barrel of the wheel inside the tire, and the band and mounting bracket have to be replaced if swapping to aftermarket wheels.
As for "rebuild kits" being for aftermarket only, this is also incorrect. Rebuilding the kits on factory wheels is strongly recommended as well for the very reason someone else mentioned; there's a little rubber grommet, and it can dry, tear, crack, and fail. And since it's holding the air inside your tire, it's importance cannot be discounted.
At 3.50/wheel for the parts, and the labor for someone to remove, "rebuild," and reinstall the sensor, it's cheap protection against the grommet failing and metal parts corroding.
ETA: So, after writing that out, I did a little looking. In Orange County, for a 2009 Acura RL (Going off the OP's information, and just randomly making up a year) the TPMS rebuild kit is 3.50. However, for the same vehicle, and presumably the same rebuild kit, in Georgia and Texas both, the kit is 7.50, and in Colorado, it's 8.00! No idea why the difference, maybe different vendors, maybe supply and demand, who knows. I can't believe I'm saying this, but for once, something's less expensive in Orange County.
