This is as close to a self-evident truth as exists in New Jersey: The key to reducing our out-of-control property taxes is consolidation.

In a small state with 565 different municipalities and 590 operating school districts, it has become an article of faith that sharing services and merging government bodies will reduce costs.

Advocates, including this newspaper, have been beating the drum for consolidation for years. The state has offered both carrots and sticks to encourage mergers and overcome the resistance of residents and officials who cling to home rule and the wasteful spending it requires.

In 2011, Gov. Chris Christie said the Department of Community Affairs would reimburse municipalities 20 percent of the upfront costs of mergers.

Last year, the state Senate adopted and sent to the Assembly a bill sponsored by Senate President Stephen Sweeney, D-Salem, Gloucester, Cumberland, that would reduce state aid to municipalities if they ignore shared-services recommendations that would save money.

Then, in January, Princeton Township and Princeton Borough became the first towns in New Jersey to fully merge since 1953. The merger is expected to save Princeton more than $2 million this year. When it is completely implemented in three years, annual savings could be as much as $4 million, well above original estimates.

The merger should be an example for other towns to follow.

But Princeton officials are now finding they may not get the full reimbursement they were expecting from the Department of Community Affairs. Princeton officials say they spent nearly $2.4 million to implement the merger, and they are hoping for about $460,000 from the state.

The problem is that - since municipal consolidation is the road less traveled - there are no guidelines as to what should or shouldn't be considered an acceptable merger cost.

No one seems quite sure whether a $340,000 bill for legal fees or $190,000 spent to upgrade information technology systems or $77,000 spent on new police weapons qualifies.

So why is it important that a community that will already save millions through consolidation get further financial incentives? Simple: The decision of the Department of Community Affairs will influence other towns that may be considering consolidation.

No one wants state money to be wasted, but the DCA should bear in mind that whatever it decides on the Princeton reimbursement will become part of the guidelines for future mergers - which the state has a legitimate role in encouraging.