Since nobody else has chimed in on this one, let me offer a few comments. It seems to me that the big advantages of structured authoring fall into a few general areas: -enhanced ability to publish content in different forms (definition A of "single sourcing") -enhanced ability to reuse content in different contexts (definition B of "single sourcing") -reduced translation costs from direct reuse of existing translated content modules -more consistent organization of information across different documents -more consistent organization of content written by different writers -more consistent presentation of similar information types -content is (theoretically) portable across a range of different structured authoring/editing/publishing tools (i.e. you're not locked into a proprietary file format)
For a lone writer, unless you have a significant requirement for single sourcing (under either or both definitions of the term), or have your documents translated into a lot of languages, the return on investment for migrating to a structured documentation environment is likely to be rather small. The big payoffs from a financial standpoint (the key ingredient of the business case for converting) stem from the reuse of content. This is a direct, demonstrable, quanitifiable benefit.