There are a variety of cost savings calculators available online. Most families will save between $1,000 and $2,000 over the course of the child's diapering life. However, this can vary from person to person and depends on the number of children you have (remember, CDs can be used for more than one kid!), what kind of diapers you get (prefolds + covers versus a stash of all $50 Goodmamas), etc.
Check out a savings calculator for your specific family here!