A bit of Mathhammer

 I was curious about the effectiveness of basic troops in 40k, these graphs compare the average number of kills a model will score against it's own point cost.

In this graph the firing units point cost is compared to a wide range of targets.

This graph illustrates how good a unit is at killing marines, for its points cost, with no special/heavy weapons.

It should be noted that special/heavy weapons always have a way better kill/cost ratio. For example, If a squad of 5 marines takes a plasmagun, the squad will have a kill/cost of 0.051(this means that they kill 0.051 MEQ targets per point you spent to buy them, per shooting phase) against other MEQ(at 24"), but without the plasmagun, they have a kill/cost of 0.037(still at 24"). Most xenos armies can't really take special/heavy weapons (Dark Eldar being a notable exception.), just keep that in mind.