I think the marginal difference you can make by adopting is probably surprisingly large. (If your expectations of the amount of good it's possible to do are well-grounded to start with.)
The difference in quality of life for a child who's adopted rather than staying in the care system is spectacularly large: it's a long time since I looked at the data but I remember it being eye-popping. And society is likely to be better as a result - there's a much greater chance of the child contributing positively to society rather than causing significant problems. (Of course, this is on average: some cared-for children grow up to do spectacularly positive things for society, and some adopted kids go on to a life of antisocial crime.)
There are way more children who would benefit from being adopted than there are adoptive parents. (In the UK it's something like 10- or 20-to-1 at the moment.) With some complex social issues, it's hard to see what the limiting factor is in improving things: not here. For looked-after children, the supply of adoptive parents is a runaway winner.
[EDIT: Woah, cultural assumptions there, sorry. From a quick glance, it looks like there is a shortage of adoptive parents in the US, but nothing like on the scale as in the UK.]
One of the big problems with organised altruism is the distance from the donor to the beneficiary: for instance, if you're trying to help people a continent away, and in a profoundly different social context, it's hard to be confident about what is genuinely improving things and what isn't. But an adopted child lives right in your house and is socialised by you, so the distance - literal and figurative - is much smaller.
I wouldn't advocate it as a life choice for people whose main goal is purely to benefit society. Parenting is bloody hard work, physically and emotionally. But some people (me included) find it hugely rewarding as well. So if you think you're likely to find it rewarding to parent, adopting seems to me like a great thing to do.
On a personal note, good luck with the process: friends and relations who've been through it have had mixed experiences. But the ones who ended up adopting report being very happy they did so.
(Bias declaration: I'm a parent of young children, and so likely to irrationally overvalue actions that benefit young children.)
Sometimes adopted children were kidnapped from their families, rather than being in the care system. I don't know how common this is compared to adoption of children who don't have families or are being abused, but it should go into the calculation somewhere.
In "The Immorality of Having Children" (2013, pdf) Rachels presents the "Famine Relief Argument against Having Children":
They present this as a special case of Peter Singer's argument from Famine, Affluence, and Morality (1972), which is why they haven't called it something more reasonable like the "Opportunity Cost Argument".
[Note: the use of "Famine Relief" here is in reference to Peter Singer's 1972 example, but famine relief is not where your money does the most good. Treat the argument as "that money would be far better spent on GiveWell's top charities" or whatever organization you think is most effective.]
It's true that having and raising a child is very expensive. They use an estimate of $227k for the direct expenditure through age 18 while noting that college [1] and time costs could make this much higher. Let's use a higher estimate of $500k to account for these. Considered over twenty years, that's $25k/year or $2k/month. This puts it at the top of the range of expenses, next to housing. It's also true that this money can do a lot of good when spent on effective charities. At GiveWell's current best estimate of $2.3k this is enough money to save nearly one life per month. [2]
But perhaps we shouldn't be thinking of this money as an expense at all, and instead more as an investment? Could having kids be a contender for the most effective charity? That is, could having and raising kids be one of the most effective things you could do with your time and money?
For example you could convince your kid to be unusually generous, donating far more than they cost to raise. Except that it's much cheaper to convince other people's kids to be generous, and our influence on the adult behavior of our children is not that big. Alternatively, if you're unusually smart, by having kids you could help make there be more smart people in the future. But how many more generations will pass before we learn enough about the genetics of intelligence to make this aspect of parental genetics irrelevant? Rachels considers the idea that your having children might greatly benefit the world, and rightly finds it insufficient. While your child may do a lot of good, for the expense there are much better options. Having kids is not a contender for the most effective charity, or even very close.
Having kids is a special case of spending your time and money in ways that make you happy. A moral system for human beings needs to allow some amount of this. It's like working for $56k at a job you enjoy instead of getting $72k at a job you like less. [3] Or spending your free time reading instead of working extra hours building up a consulting business. Keeping in mind both the cost and that on average people don't seem to be happier parenting, if having kids is what would make you most happy for the expense in time and money then it seems justified.
(This is how Julia and I thought of it when deciding whether we should have kids.)
I also posted this on my blog.
[1] College is currently in a huge state of flux. Advertised costs are rising far faster than inflation as colleges realize they can get away with near perfect price discrimination in the form of "either pay the extremely high sticker price or give us all your financial data so we can determine exactly how much you can afford." At the same time online courses and mixed models are getting to where they can provide much of the value of traditional lecture courses, and in some ways do better. I have very little idea what to budget for college for a kid born now; likely costs range from "free" to "all you have".
[2] Rachels uses a much lower number:
Their Brenzel citation is to the Vaccine-Preventable Diseases section of the DCP2. The $205 number is "Estimated cost per death averted for the Traditional Immunization Program in Sub-Saharan Africa and South Asia" in table 20.5.
[3] This is a $16k difference, which comes from taking $500k over 20 years and dividing by two for the two parents, and then adding some for taxes. Though the earnings difference is likely to last more like 40 years.