I personally think if you're a local artist $100 a month can still be significant scratch to try to consistently come up with to market what you do. However, what's the cost of not marketing? (That's a bit of a rhetorical question).
One way to build a budget each month to market your music is to play a few shows each month and reinvest any revenue from gigs back into your music marketing efforts. Take 15%, 25% or even 50% of what you earn each month from gigs and put it toward monthly marketing costs.
As this blog post suggests you can get a lot of marketing reach for your SoundCloud song post on Facebook through Facebook's very targeted paid posts.
Definitely something to consider.
Posted by: Nick Venturella