Data on costs of two-year colleges in the US, by state?
From people's comments and more web-surfing, I've found enough info to take some kind of stab at answering my own question.

California has the lowest community college tuition rates in the US. It appears that every state has some kind of community college system. Most states charge about 3 times more tuition than California. Roughly speaking, in most states a student paying full community college tuition (because they don't qualify for free waivers) is paying about 60% of the cost of instruction, with the other 40% being a subsidy from the state government. In California, it's more like 20% and 80%. 

It's extremely difficult to give a clearcut comparison of costs. Various states have their own baroque rules. E.g., in California about half of students get fee waivers. These waivers used to be called BOG (board of governors) waivers, but are now subsumed under the College Promise program.

As an example of how complicated this can get, Pennsylvania has a system where some school districts choose to contribute to community college funding and others don't. If you live in a district that doesn't contribute, you pay higher "non-sponsored" tuition to attend a community college. The highest per-unit in-state tuition at any community college in the US seems to be $318/unit for non-sponsored tuition at Community College of Philadelpha; this is about $10,000/year, or about 7 times higher than California tuition.

College Promise is extremely complicated, and is implemented in different ways by different states. It's been sold to the voters as "free community college" or "free first year of community college," but it's actually both more and less than that.