This is an interesting question and the answer is complex.  If availability of funds is unlimited then clearly spending any amount to reduce any bad thing is warranted.  In reality, resources are limited and decisions regarding optimal use of funds must be made.  For every decision to spend money on one thing, we spend less on something else.  In absolute terms, terrorist murder comprises a very small percentage of total murders, yet we spend far more money fighting terrorism than we do fighting other murder.  To do so is irrational, but that is who we are.  

More generally, I think that murder is reduced not through conviction and sentencing, but through broad public acceptance and support of the value of human life.  When we as a society treat people as disposable, dehumanize them, call them pond scum, drop bombs on them, or kill them via police, we teach individuals that it is okay to solve personal problems through murder, robbery, etc.  We reap the fruit of what we sow.