DHITW built us a shade house on the cheap. He used mostly found lumber, wire mesh, shade cloth and a whole bunch of burlap sacks we got for free from our local nursery. It cost under $100.00 to make our shade house. If you bought all the lumber new it might run you closer to $200.00.
To start seedlings for fall outside in Texas, you are going to need a really shady place. We wanted a shade house that was large enough to walk into and with different areas for potting and transplanting. We also built wooden boxes that fit underneath the shelves for worm composting. Then when we water the plants in the shade house, the water will leak down and water the worms. Worm casting are amazing compost and this keeps the compost and the seedlings in one convenient place. Another plus to building your own shade house is you can also design it to transform into a greenhouse. In the winter, we will take down the shade cloth and burlap sacks and cover the wire mesh with plastic sheeting to keep the plants warm.