There's no reason you couldn't start small. Find some cause you support or someone you know in greater need than yourself and contribute what you can. If you feel you can't do anything now you probably will always be able to find a reason you can't in the future.

> I'd pledge if I had any hugely significant sum like the others in the pledge

Lives in the developing world can be saved for less than $1000/life (that is, something like $20 per disability-adjusted-life-year). You personally could save dozens--perhaps hundred--of human lives, which I consider to be hugely significant.

but for how long?

I quoted $20 per disability-adjusted-life-year (http://en.wikipedia.org/wiki/Disability-adjusted_life_year). That means for $1000 you could allow one child who otherwise would have died an infant to live into old age, extend the lives of 10 people by 5 years, etc.

I earn a pretty good salary in a developing country. 'Pretty good' is quite relative and would be minuscule compared to salaries in the developed world. But - I live a comfortable life here, and 10% of my monthly salary I donate is sufficient to support 2 kids for schooling, lodging and food.

So, yes, starting small means a lot, and it doesn't need to be a huge amount at all.

their whole lives