If you have followed discussions in social media, blog posts by prominent translators, and presentations at translation conferences, you've probably heard that subject matter expertise is the essential key to getting better clients and better rates as well as attaining higher translating speed and higher personal satisfaction. This means not simply defining your "market niche," but also becoming so immersed in the subject area that you can speak the language of that subject with as much fluency as someone originally trained in that field. Courses aimed just at translators are too narrow: they tend to focus just on terminology in the field, but terminology without context will never make a translator fluent enough to claim subject matter expertise or to deal expertly with translating in that field. If you want to corner the market in a particular subject matter, become the go-to person for that field, and command top dollar, you also have to know the basic history of the subject area, its major debates and current concerns, and the style of writing used in that field.
Although self-directed reading is a good start, there is no better way for gaining in-depth knowledge of a particular specialty than directed learning under an expert in the field in tandem with others who are learning. It used to be that the only way to get access to this learning was through physically attending courses at colleges or universities. This meant either going deeply into debt to pay for the privilege or sitting in as an auditor (if allowed), but having no chance of getting feedback on your work from the instructor or proof that you had attended and done the assignments.
MOOCs have changed all that. They are an incredible resource for learning a subject area, whether at beginning, intermediate, or advanced levels. Even the most basic ones go far beyond webinars or so-called "courses" aimed just at translators. MOOCs range from simple filmed lecture courses (which, if the instructors are dynamic speakers, can be mesmerizing) to multimedia extravaganzas delivering content through several channels, reinforcing learning and meeting many different learning styles. Moreover, a translator can get subject matter expertise and hone their linguistic abilities at the same time by taking MOOC courses in their foreign language, effectively doubling the benefit! And how can you beat the cost?! As Eleonor noted, they are usually free to take, with small fees for certificates attesting to your having taken the course and passed the graded assignments. What's more, almost all professional translator associations will accept these MOOC certificates as proof of fulfilling requirements for continuing education units (CEUs) or continuing professional development (CPD). They are a lot cheaper than all the other well-trodden routes (paid translator webinars/courses, conferences, etc.) for fulfilling these requirements.
Any translator who is not taking advantage of these MOOCs is losing out on the best avenue to come along yet (short of getting an advanced degree on campus) for upgrading their skills, surpassing the competition, demonstrating subject matter expertise, and making a better living with a more sustainable career. I can't recommend them highly enough.