This is a great site for practical world building experience. It isn't related to X3D but the advice is first rate. I found this on Bob Sutor's blog.
There are some very interesting analyses of costs to build worlds. I wonder if the advantages of standardization would reduce the costs. Note his comments about the high value of hand building over algorithmic generation for content stickiness. I couldn't agree more. I do wonder about the cost advantages of relying on XML, X3D and standards for server communications and if the cost advantages interfere with the need to produce a better or different world.
I also wonder if game-play centric worlds are a very different cost problem than the virtual world models.