I'm in Houston for 3 weeks at a time. I get Hotel and Car Rental covered, and $30/day. It's pretty slim, but I got the Hotel to throw in free breakfast. Subway lunch is $5.00 which leaves usually $20.00 for dinner. I usually charge the other $5 on random stuff like Energy Drinks or food for my Hotel room.
You should be able to get more than $30, especially if you are travelling on contract work. I'm in training, so my company cannot just charge it out to a client.