Price per HP, for me, is more of a way to avoid price 'traps'. For example, if there's two modules that have similar functionality, but one actually works out cheaper in price per HP when space isn't a primary concern, then that's a decider. But when space becomes a key criteria, then price per HP can slide a bit. For example, I know that two Doepfer A-171-2s come in at a higher price per HP each than does a MakeNoise Maths, which is essentially the same setup with a couple of extra bells and whistles. But if 16hp gets the priority over 20hp, then the Doepfers get the nod. Conversely, if I can fit 20hp, then the lower cost rules. It's not a hard/fast rule that gets followed when I do a build, but more of an indicator of potential directions and, often enough, a way to whittle down possible choices among similar modules.