Dallas Fort Worth a metroplex so they have an identical cost of living. For free majority of national statistics, they're treated as one city. Unless the state has recently redone the Texas-only cost of living analysis, you're not going to find a seperate value for Fort Worth.
The way it breaks down, per the 2015 cost of living analysis, if you're single, Dallas is literally less than 100$ more affordable than Houston. If you have a child, Houston is several thousand dollars more affordable than Dallas.