I know a place called Shady Grove MX, in Shady Grove Texas. The cost is 10 bucks. Also, in Shady Grove, there's another place you can go for free.. it's really dangerous though. It's all the way at the end of Shady Grove Rd. Then, there's one on the Texas/Oklahoma border called Red River.. I don't know a lot about that one.
Dan is correct. If you like MX, you need to check out the North Texas Smack Pack. There is a MX group going somewhere almost every weekend. This is Blackhawk468's home chapter and there is a link to contact her with in the roster.