This may not be a useful answer for you depending on your situation, but the best money I've spent on professional development has been funding sabbaticals between gigs where I engage in deep study on topics that are harder to learn in small doses.
This is a useful answer, especially because I don't think most people consider or make this decision mindfully. Sabbaticals can be an important option not only to advance your professional development but also to avoid burnout.
And I always hear people who consider it, but nine times out of ten (or probably more), they drop the idea for a copout reason (I met someone / need to save money for X / this job is too great to leave)
