Those of you without kids probably have trouble imagining just how many diapers a single baby goes through, and that's not to mention families with many kids... In the early weeks and months, it wasn't rare for us to change diapers 10-15 times per day. All of this adds up quickly in the landfill (though for most people, out of sight, out of mind applies), but also in the wallet.
How much you can save with reusable cloth diapers depends on a lot of factors, so I won't attempt to do the math here. But for us it was definitely worth it. They cost more upfront when you buy them, but once the initial expense is passed, they provide a nice return-on-investment.
One estimate puts disposable diapers in the ballpark of $1500-2500+ while cloth diapers are closer to $500 (and can be had even cheaper second hand). So we're talking about multiples more, not just a few percents.