on a personal level i'd suggest if possible to have a garden ,or if possible in your area and you don't have a garden buy a garden plot somewhere. this is really helpful if you make it nice for insects and birds. this is someting generally low cost, but does take time and effort.
in my area if you don't have a garden you can get a plot for about 100-200 euros a year. so thats dirt cheap to make a bit of impact.
cleaning in your area / natural areas is also good. walking around with a bag and collecting plastics and other trash left by people which might impact the local wildlife / environment.
those are generally things that do cost time, but not a lot of money, i'm all for activities in your life opposed to just trying to solve it by throwing money at it.
like a silent personal protest :) it's highly effective. for example in my parents garden there are a multitude of bird families who come there every season to live or in passing on their migrations. additionally we see a lot more butterflies and bees again since they started selecting the right type of plans and flowers for the local species.
on the other comment, i don't think 'overpopulation' is a problem. just how the population behaves. but i suppose that's a matter of opinion formed by where you live. i don't live in a huge city, so that might skew opinions on that matter a bit.
another idea is giving free workshops to people how to live consciously and environment friendly. a lot of people would like to do this kind of things ,but don't have the know how to do it. an hour or two with the right content of workshop can totally convert someone to living more environment friendly the rest of their lives. they will also give those insights to their childrent, propagating the effect.
all in all its more a mindset problem than anything. people are not conscious of this problem, which keeps it in place.
if you really want to throw money at it. try funding and helping researchers / students working on renewable / cleaner living solutions. in most countries there's some university or research group focusing on this topic. they could always use some money if you have some to 'burn' ;). 2500$ a month can get them a bunch of research equipment and buy them time to spend with that equipment to research. some people don't even earn that much, so you could fund someone to spend all of their time on this topic.