I now have heard of a number of people who are making money from mashups, but not from the mashups themselves. Instead they’re making money as the ones building the mashups for others on a work for hire basis. Mostly as a side job or small consulting/freelance gig. Model generally goes: build a few mashups of your own (“resume mashups”) and then hang a shingle as a mashup expert, perhaps within some sub-specialty like Google Maps, Amazon, or eBay. Some of the developers are quite good. Not a big money business of course, just a new niche.
Who’s paying for this? Media companies: see the recent NYTimes mashups or HBO’s. Real estate companies: few real estate sites don’t have some flavor of map these days. Recruiting sites: where’s that potential employer located? Independent developers can often help build these more quickly and cheaply than in-house staff.
Another market is the API providers themselves. A number of APIs have lingered on the vine because the provider dropped the ball by not leading the way and creating useful examples of the API’s capabilities. Just publishing an API spec is not always inspiring. If the API provider lacks sufficient in-house resources or time they can contract-out mashup development.
Clearly a small market today but as more and more APIs become available we can probably expect to see an increase in “web API experts” for hire.