Knowledge is by far your best investment. But as far as a purchase... my cage was my most useful tool. We pitch, bat, soft toss, front toss, and overhand throw in the cage... never chase a ball again
Video cameras are a great tool as well, but for just viewing and analysis, look into a free phone app called "right view pro" it can be played back in slo mo
Don't underestimate the value of a pitching machine, we found ours on craigslist for $400 (single wheel jugs) we also use it for grounders, deep fly balls, one hoppers to the first baseman, and catcher drills.