Should the police put more or less effort into solving murders among criminals?

Equal.

All should get equal treatment but it seems obvious that the cops would care less about having one less drug dealer around.

No. Every murder requires equal amount of attention and time. If we started discriminating about whose murder is more important to solve, then I guess we'd just be discriminating.

Each should get equal attention, but in real life I'm fairly certain that's not the case. Although not working for the police I can only go on what I see and hear.

This is relatively easy. I think whichever case contains the most factual information and evidence pertinent to the crime should be given some priority, more than cases that are completely cold. I wouldn't use the word 'equal' as each individual situation has its own set of circumstances. Though I am sure, that in the real world...the businessman's case would be given greater consideration. And then it comes down to public figures vs private citizens. Internal pressure/ politics, family pressure and several other deciding factors also contribute to increased investigative exposure. But I am not saying that criminals should be placed at the bottom of the barrel due to a extensive criminal history, either.

Other -- we should maintain the level of effort into solving murders among criminals. Neither give them more nor less level of effort than they currently have. The police more or less have it right.

Remember, people (in the US at least) are innocent until proven guilty, and to get proof, you need witnesses. The problem is, when criminals kill other criminals, the witnesses are other criminals as well. Not exactly the most credible bunch, especially when sweetheart plea arrangements are made in exchange for testimony. It makes sense to put less effort into cases that you have less chance of winning, where the guy will walk anyway.

Any criminal capable of killing another criminal is capable of killing just about anyone and is still a danger to the common good. The justice system doesn't exist to vindicate the families of victims, but to maintain order... Not that it's doing a very good job.

Murder is murder. It is a horrible action in any situation. It is where someone has just killed another human being. Even though a situation may not contain obvious criminals, it is still a dreadful crime and needs to be taken seriously and addressed to.

All crimes should be investigated with the same vigor. Presently the police treat crimes on criminals, the poor, and those with little money or political power in a very poor manner. Only the rich, those with political power, or crimes on other cops get real investigation. Most crimes are solved only because the criminal is standing there with a smoking gun, or very stupid and leaves a ton of evidence, otherwise the brain dead cops couldn't solve a thing. One of the great fallacies about gun control, the cops are not there to protect you if you are a victim of crime you are on your own. The cops just show up and put crime scene tape up and outline the body.