Good point, 2500 USD is pretty cheap for bribe...
Some of the 'pricing' that goes into a bribe are
A) chances the person taking the bribe will be caught accepting the bribe.
B) chances the person taking the bribe will be caught doing whatever they were bribed to do
C) risk level after considering any negative consequences for the bribed associated with B
D) actual difficulty executing B
E) how likely the briber can expect his payoff to secure the desired outcome
Lets look at this situation:
Group of officers starts a "Fraternal Order of Police" or something similar its ostensibly a charity for injured officers but also throws some fancy thank you and holiday parties for the force (the payoff). They pass out window stickers to contributors at certain levels. They know this helps because people like show off how generous they are (legit reason many charities do this (the cover)), they also know some people will cynically believe it will buy them special treatment and this badge is how they prove their entitlement. The officers with a nod and a wink agree to actually provide this special treatment because they think it will increase the donations leading the fancier and more frequent parties.
Will they get caught? Not very likely unless someone does anything very stupid. They time when they collect the funds vs the time when they commit the act are widely separated. They act itself is in the negative. Not pulling someone over in the first place because you saw an FOB sticker in the window creates no audit-able event. Even fairly honest members of the public are unlikely to call the mayors office and complain that they just blew an officers doors off and he sat and did nothing. If there are lots of people around and the behavior is egregious they can pull someone over and warn them, none of the whiteness are likely to be able to tell if a warning or a ticket was issued.
C) The risk is low because the odds getting caught are low and even if someone suspects their shot at proving anything is almost nil. It will be very hard to make any conspiracy charges stick, the worst the will likely happen is officers might be dismissed for under performance. Proving negatives are not easy; especially when there is already a discretionary element to writing tickets or not in the first place.
D) Could not be any easier to execute, in fact its probably easier than doing their job correctly.
E) Not every officer, likely not even most, will be in on the conspiracy, the payer cannont know for sure he won't get pulled over by an honest cop.
So considering the situation the 'price' of this bribe should be low.