Just as an example, here's the tuition for undergrad VS. grad at UWM. Keep in mind that undergraduates take 15 credit hours VS graduate 12 credit hours.
2012-2013 Undergraduate Tuition
Resident Undergraduate Tuition* Based on 12-18 credits both semesters $9,380
Non-Resident Undergraduate Tuition* Based on 12-18 credits both semesters $19,088
2012-2013 Graduate Tuition
Resident Graduate Tuition Based on 8 or more credits both semesters $11,524
Non-Resident Graduate Tuition Based on 8 or more credits both semesters $24,412
So if we figure it out by cost per credit, going by minimum credits for full time.
Undergrad in state = 391 per credit
Out of state = 795 per credit
Grad in state = 720 per credit
Out of state = 1525
Almost a 100% increase per credit.
Most of the other schools I've looked at are similar in cost.