It isn't just about pot tax revenues.  Perhaps the article might also have looking into the following items:
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1.    Have law enforcement costs decreased?  Have court costs decreased?
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2.    Have the police been able to re-direct their resources to higher priority work?
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3.     Have fewer people gone to jail? (each prisoner costs US $100,000/year.  Reducing prison population therefore represents huge savings).
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4.     Are there fewer people employed in the non-legal market? (i.e., less vulnerable to exploitation; more likely to be paying income tax and payroll tax; more likely to have better health care associated with employment).