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.
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)