Q:
Won't it be too expensive?

A:
On a per hour basis an external author will cost more than employing your own dedicated staff, but you won't be adding to your overheads and you won't be paying a salary either.

You simply engage a Technical Author when you need one, e.g. a new project or to supplement existing staff when there is a temporary upswing in work.

If you're a small company and cannot afford dedicated staff, then it is less costly to engage a specialist for short periods.