Welcome to the second open thread on the Effective Altruism Forum. This is our place to discuss relevant topics that have not appeared in recent posts. Thanks to an upgrade by Trike Apps, each time you visit the open thread, new comments will now be highlighted!
A strong consideration in favour of donating on that sort of modest wage is that it gets you into the habit of doing so, rather keeping on putting it off until you're richer. It also makes you better able to influence others to donate.
Also, an admittedly cursory look at Wikipedia suggests that the median adult income in the US is $24k/year, which'd suggest that telling these people not to donate would exclude half of all adults. (I expect that $24k/year is not the most relevant figure to use here, but the point stands that it's easy to underestimate how wealthy we are even compared to others in the developed world.)