We just moved into our custom home in Bellaire a few months ago. We owned the lot and found the builder. In my opinion, was was true for us, you will save a lot of money by owning the lot first. I think there are several reasons, with the obvious being that the builder isn't having to pay the financing on the lot/construction. Another reason is that finding lots in prime Houston locations takes a lot of time and money. If you're buying a spec home, you're essentially paying for the builder to find the lot for you and do all of the work.
Another benefit of having your own lot that can save you money, that we took advantage of, at your own risk, is that builders will be competing to build on your lot. The market is down since we started building, so this might have changed, but we found 2 builders that we liked, made sure they were quoting apples to apples, and basically had them bid against each other. Once we had the lowest bid, the other builder said they would go lower, and then the builder we slightly preferred agreed to go a bit lower. At that point we knew the price was at the absolute rock bottom, so we went with them. This strategy can have risk though, as our builder was operating on slim margins. They get pissy when things cost them more than they thought it would during construction.
Keep in mind, most builders gross 15-20% over the cost of construction. Many will do cost plus 15% or cost plus 20%, which reduces their risk. I wanted a fixed cost (less risk for me), and our builder did that. Not sure what prices have done over the last 2 years, but when we contracted, you could probably get a very basic custom house for $125/ft. With moderate upgrades that most would consider a very nice house, you'll be at $150/ft. I'm sure you could get to $200/ft, but damn I can't imagine what you would dream up to get to $200/ft.