The argument is over a 0.6% difference in their pay rise (1.9% instead of 2.5%). It is not the £30m cost that is the problem for the government, it is the signal it sends out to the other 5m public sector workers. If the most well paid and least productive of public sector staff deserve an above inflation rise, then the rest will demand a hefty rise as well. Perhaps the police could follow the example of teachers and accept a three year deal.
Of course, you could argue it is the principle the police are fighting for - the govt have previously honoured the independent review. But did the police complain when their pay went up faster than recommended? Nope.