I was contemplating this as I was folding diapers today, and thought I'd share my findings.
Let's assume your child is in diapers for 3 years. That's 36 months. Disposable diapers, assuming you change your child 8 times a day (the same as you would for cloth diapers), would cost an estimated $40 per month. That's $1440.
A dozen infant sized cloth diapers costs about 20. I like to have 3 dozen, which means I wash about every fourth day, so that's $60. A wool cover, if you knit it yourself or have Grandma do it, costs about $6 in yarn. It's good to have six of them - $36. So that's $96 for about the first 4 months, approximately. Baby will eventually need larger prefolds, about $75 for three dozen, and you'll need 6 covers in medium and large too, maybe costing $9 in yarn each, so $54 and $54. So adding all that up.... $135 for prefolds, $144 for covers, a total of $279.
Of course you can spend lots more than that - fancy all in one diapers, fitted diapers instead of prefolds (running $8 each if you buy them new, as opposed to about $1.65 for a prefold.) You can buy expensive yarn, or pay $25 each for someone to knit them for you. Of course, if you don't have someone to knit them for you for free, you can buy polyurethane covers for about $8 each instead. So there are a ton of variables with cloth diapers - I used my own system as my example.
The great thing about cloth diapers is that you can use them for each kid. If you have 3 kids, and reuse all the same diapers, that means it costs you about $93 for each child - as opposed to $4320 if you put 3 kids in disposables. You may have to replace a few things over the years, but most everything should hold up fairly well. And then, just imagine what you'll save in Pledge wipes when you turn all your old diapers into dust cloths after you're done with them. :-)
And of course the deciding factor for many moms - they're just flippin' cute!