Because I do not use disposable diapers full time, I don't know firsthand what it costs. I know some mom's who find good deals online or use coupons, and they do save a great deal. I can tell you about how much I've spent cloth diapering, though.
When Izzy was born, I had $150 dollar limit to spend on cloth diapers, so I went with the cheapest diapers available - prefolds and covers. I bought 18 prefolds, 8 covers, a diaper pail liner and some wipes with that budget. As time went buy, I splurged and got a few fancier diapers and some more covers. When she got too big for those diapers, I bought some more and overall I may have spent $300-$400 total.
When Korban was born, I could have easily used what I had plus a few new covers (they start to lose their water-proof liner eventually), but I had just started as a consultant so I used my discount and some of my income to try out several types of diapers, so I spent a hundred or so there.
Now that we're getting ready to diaper baby number three, I know that I love the prefolds and covers I initially bought, but after two kids, my covers have pretty much had it. I'll be replacing the covers, but the prefold diapers themselves are still fine. I've only had two of those wear out on me.
I don't have an exact number for you, so I'm going to estimate way high here. After having cloth diapered 3 kids to potty training, I expect to have spent less than $1000 dollars. And that's including the cost to launder them, which is a couple bucks a week for water, soap and electricity.
The last estimates I saw for diapering a child in standard (not the natural brand, which costs more) disposable diapers showed a cost of $1500-$2000 per child. So, for three kids, that's $4500-$6000.
Why yes, I'm quite proud of my savings of $3500-$5000 dollars.
Here is a link to the Zukababy cloth diaper info page. I love their advice, and I'm a consultant for them.
Also, here are links to the prefolds and covers I ordered, if you are new to cloth diapers and need a visual.
And, here is a link to my past posts about how I cloth diaper, for your reading enjoyment.
Look for another post with more details about the diaper show!