If you did an illegal U-turn, you might consider doing the right thing and paying the actual fine. It was not going to be much more.
By 'bribing' the PN or AMET you are rewarding them for behaviour that they should not exhibit IMO. You are adding to the impression that 'bad behavior' pays and that those without recourse are there for the taking, instead of protecting. You are Canadian, I believe in Canada you would have paid the fine and not tried to bribe the police.
The behaviour of 'bribing' adds to the problems with law enforcement that everyday people here experience. You might not be affected by a DR$ 500 bribe, but if you will take a moment to think about what it would mean to the watchyman getting off duty who earn DR$ 10K for 20 days work - if he is asked to fork over DR$ 500 for no reason, he just lost a whole days pay, what are his children going to eat that day? And then we wonder why the watchymen betray our confidence and invites his ladroncito cousins to make some money.
Please, if you want to contribute to making the DR a safer place to live, where the rule of law is increasing rather then decreasing, we, as foreigners and Dominicans alike, should stop feeding the corruption where we can do so. We should set an example where we can, try to hold the PN and AMET to their standards and pride where possible.
I am not talking about putting up a fight when the PN stops you on a back road at night where there is no-one to see, but there are often circumstances where we can decide to do the right thing with little risk to ourselves.
As an aside I know the PN and AMET are underpaid. The resolution to that is not in robbing those they are sworn to protect and loosing the respect of the populace... No the resolution lies in the govt spending less on botellas and jeepetas and more on salaries for their professional services, so they can do their jobs and don't have to turn to highway banditry.
PS: In your case the lack of license of course played a role and you did well to avoid an offical citation. But what would have happened if you got into an accident? You know your insurance would be invalid right? You know that you would be liable for the other party and their whole future earning potential right?