This instructable describes how a repair can be made for less than $15 (depending on what tools and parts you have on hand.
Step 1: Thread Broken Stem
The tire stem of the TPM needs to be threaded. I thought at first that adding outside threads to the unthreaded part of the broken stem would allow me to screw on a new tire stem. I realized after I did this that this approach wouldn't work since the retainer nut needs to fit over the stem. This was a unnecessary sidetrack.
The approach that worked is to tap the inside of the stem and screw in a new threaded tire stem with matching threads.
I had a tap and die set that I bought at Harbor Freight for $8. The best matching tap size was 12/24. The ID of the broken TPM was 0.166".
I cut out the tire stem from an old bicycle tire inner tube. After stripping off the extra rubber, the OD of the unthreaded portion was 0.236". I used my grinder to reduce the diameter of the brass a little bit until the 12/24 die would snugly fit.
I used a little thread locker to make sure the brass part from the tube fit leak-free in the aluminum TPM.
At this point, the assembly could be re-installed in the Tundra wheel.