When my daughter took one online course one semester (her choice, not required) it actually cost MORE, not less. She paid for classes by the credit, so she had to pay for the 3 credit class. She also had to pay a ‘distance learning fee’, and because she had 12+ credits in on campus classes, she had to pay all the on campus student fees for that semester. If she’d been all online, she wouldn’t have had to pay those on campus fees.
If a school is offering the same number of credits and the same degree in the end, why wouldn’t they charge the same tuition? It’s a business and it costs them the same for the professors and all the resources they are providing like library access, advising, administrative support, producing transcripts.