You can get a lot of things out of martial arts training, but I don't think you can get many of them in a month. If you want to have a better chance in self-defense situations, you're not going to gain the skills or the habits you need for it until much later. If you want self-discipline et cetera, you're not going to get much of it from learning falls and a few basic escapes. If you want to get in shape, you'll barely have started. If you just want to have fun and feel badass... okay, it's plausible that you could do that in a month, but it's not going to last once you stop training.
I honestly feel that "learning to fight effectively" is kind of a red herring here. Martial arts does make you better at fighting -- but few people, martial artists or otherwise, are good at real, no-holds-barred fighting, because parts of that skillset are so dangerous that they basically can't be gained without being repeatedly injured or worse. But that goes for the bad guys, too. Self-defense largely isn't about beating your mugger or whatever in a serious fight; it's about deterrence, mainly through signaling comfort with conflict situations and by giving you the skills to deal with strongarm tactics short of serious fighting.
And I wouldn't recommend taking martial arts classes primarily for self-defense anyway, at least if you live in a Western country and not in the worst parts of a high-crime city.
What can I purchase with $100 that will be the best thing I can buy to make my life better?
I've decided to budget some regular money to improving my life each month. I'd like to start with low hanging fruit for obvious reasons - but when I sat down to think of improvements, I found myself thinking of the same old things I'd already been planning to do anyway... and I'd like out of that rut.
Constraints/more info:
Background:
This is a question I recently posed to my local Less Wrong group and we came up with a few good ideas, so I thought I'd share the discussion with the wider community and see what we can come up with. I'll add the list we came up with later on in the comments...
It'd be great to have a repository of low-hanging fruit for things that can be solved with (relatively affordable) amounts of money. I'd personally like to go through the list - look at candidates that sound like they'd be really useful to me and then make a prioritised list of what to work on first.