There are 2 business models here.
One is where the expert resides in a big organisation (for example an oil and gas engineer). The ‘Software for Domain Experts’ business model is to make software tools for these people to use.
The other is where the expert works as a consultant or has his own small business to provide a task (such as a recruitment consultant or specialist advisor). This expert can build specialist tools (perhaps working together with progammers) to help him or her provide this service.