Most martial arts schools I've checked out have monthly tuitions in the $50-150 range; so on the low end, that's at least $600 tuition. You won't get much benefit out of just 1 class a week, but 2 classes a week at an hour & half a piece (travel, changing, recovery + 1hr class ) times 3 weeks a month times minimum wage is $72 a month or $864 a year. So paying for and going to the classes is going to cost you >$1500 a year. That could be a large chunk of what it takes to get out of poverty, for example, that alone is a good chunk of many community college tuitions.

Combine that with the low absolute rate of crimes and the unknown effectiveness of standard martial arts for dealing with said crimes...