We are currently invested in Illinois 529’s Index Equity Portfolio. It’s expense ratio: 0.12%
If we were to buy individual Admiral funds from Vanguard, the overall expense ratio would be: (57 * 0.04 + 36 * 0.11 + 7 * 0.12) / 100 = 0.07%
This is surprising because I originally assumed the former to be cheaper.