Wisconsin, prior to Scott Walker and Act 10, had an exceptionally public sector union, which had created a health insurance benefit that had two facts the law aimed to address
Union employees contributed less towards their health care
Prior to the law mandated changes, the average contribution was small
In the 2010-11 school year, Forbes said the average employee contribution for health premiums was 5 percent for family plans and 3 percent for single plans.
The law mandated employees contribute more (with a goal of 12%)
[E]mployees at more than 100 districts – approximately one-quarter of Wisconsin’s 422 public school districts – paid less than 12 percent towards their monthly health insurance premiums in the 2017-18 school year. Still, that’s a far cry from 2010-11, when 43 percent of all districts paid the the entirety of their employee premiums on single plans every month. Today, just 6.4 percent of districts pay employee premiums in full.
Health insurance could be bid out
Apparently unions could dictate which insurance could be purchased
The analysis found that Wisconsin saved $3.36 billion by requiring government employees contribute a reasonable amount to their own retirement. The analysis also estimates local units of governments saved an additional $404.8 million total by taking common sense steps like opening their employees’ health insurance to competitive bidding. Milwaukee Public Schools saved $1.3 billion in long-term pension liabilities, and Neenah saved $97 million in long-term pension liabilities in addition to other savings.
Nobody goes into detail as to what that renegotiation entails, but it's quite possible the unions had their own deals with health insurance providers, and mandated their use in contract negotiations. Since the state picked up all premiums in many districts, there would be little incentive to select economical plans.
Remember, the net effect of Act 10 was to weaken collective bargaining in Wisconsin. With weaker unions, many government agencies were free to make decisions that the unions likely would have objected to prior to the law.