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I think a state directory could be a great idea depending on if you are looking to recieve traffic based upon local searches.
I would judge the value of the directory on its position in the search engine.
For your example I would do a search for "texas web designer" and see how they come up. If they place well, and you do not I would consider trying it out.
You could also further research the site by checking out their "traffic volume" on alexa.
Usually with the smaller niche directories, it is highly targeted/converting traffic so one sale is usually enough to pay for it. You might be able to negotiate a free trial month and see how well the listing performs... just make sure you know how to add a tracking URL as well as watch it in your log files.