Is it normal for graduate tuition to be much cheaper than undergraduate tuition?
The standard full-time undergraduate course load is 16 credits per semester (32 credits per year). (See here: the four-year undergraduate degree requires 128 credits.) The flat-rate cost for the example you have cited comes out to $1539.375/credit.

The standard full-time graduate course load is 9 credits per semester (18 credits per year). The flat-rate cost for the example you have cited comes out to $1538/credit.

For the school you have used as your example, the per-credit fee for part-time students is $1538/credit in most cases.