I took these ideas quite seriously a few years ago when I posted on the topic, and I still do, but my attitude has changed somewhat. I used to see extreme frugality as an extreme moral imperative. Since then, I realized that as you mention, high income is probably more important than low expenses. I realized that I only had a limited amount of willpower to work with, so I chose to apply my optimization pressure in favor of increasing my income rather than decreasing my expenses. (I'm still quite frugal, but it's mainly to keep my burn rate low. If things go well, I won't be like this forever.) I also realized that I was experiencing a great deal of internal turmoil from complaints from my selfish side. So I began to act on an average of the impulses from both sides: my life's purpose didn't really change, but I'm much more open to making concessions to my own self-interest.
Of course, if D_Malik is to believed, it's possible to develop quite a high level of willpower in oneself through progressively higher willpower expenditure:
http://lesswrong.com/lw/5b8/insufficiently_awesome/3z5r
Clearly this is a complicated issue. People may differ, and it's all about understanding and exploiting your brains spaghetti code. I'm thinking of messaging D_Malik and getting him to teach me his method. Then maybe once he's made it communicable one of us will write a less wrong post about it.
To end on a somewhat sad note, I've noticed a trend towards greater selfishness as I grow older. It's too bad that the trend doesn't go the other way, since old people have all the money.
"I only had a limited amount of willpower to work with, so I chose to apply my optimization pressure in favor of increasing my income rather than decreasing my expenses."
This is very sensible.
From Yvain's 'proposal' to measure money in dead children:
This makes sense to me, to a limited extent. You can spend money for your own benefit or to help others elsewhere, and there really are people who wouldn't have to die if you would forgo some luxuries. Making this tradeoff more explicit ("we're looking for an apartment costing no more than six dead children annually") might lead some people to greater generosity. It's a way of abstracting compassion.
Two things worry me, though. The first is that there's a big focus on spending here [2], but increasing earnings deserves more focus: getting a raise or a new job that added $10K to my salary would let me keep more children from dying than would reducing my spending on myself to zero. [3] The second is that thinking of all your purchases in terms of dead children is likely to make you miserable. Not just that, but miserable to little gain: you still probably spend almost as much money on yourself, you just feel more guilty about it. Much better, I think, is to pick a rule for how much to give and then apply it to money as it comes in. That way each purchase has no effect on the number of deaths you're averting.
(Note: I also posted this on my blog)
[1] The current number is probably closer to $2K.
[2] Maybe this is because it sounds weird to talk about salary in terms of dead children? ("I wonder what job earns me the most dead children?") Perhaps for earning the unit should be the "undead child"?
[3] In 2011 Julia and I lived on $18K for the two of us, not including taxes or health insurance.