Sure, if you are in a competition to build the highest peak HP car for the least amount of money the HP/$ is a very important metric. However I don’t think any of us are doing that.
When looking at power adding
mods the peak HP gain is the “marketing number” but that’s only a small part of the power equation. How the entire HP and TQ line moves is important. If the peak HP moves by 20hp but only at 6k rpm then you’ll only see that increase in very rare situations. But another mod gives you 10hp through the entire power band where you will see it all the time. If they cost the same amount, which one is better? I say the 10hp mod but it doesn’t have the better “HP/$” metric.
Beyond HP,
mods like an intake, exhaust are about more than just HP. Look at the number of posts about drone on exhausts, people care about the sound just as much if not more than the power increase. Intakes generally will improve throttle response as well as help other “air flow”
mods (intake, turbo, downpipe, exhaust).
Then there are
mods that don’t even add power but make the car faster. Suspension, tires, brakes.
Edit: One other example I forgot about, intercoolers. Once a good one is developed it will probabaly be about $1k and advertise 10-15 HP gains. Has a terrible HP/$ metric but will be vital for anyone that wants peak HP after more than 5min of running hard in 80+ degree temps. It’s not a mod about making power it’s about retaining power.
Again my point was that trying to use HP/$ is pointless, even more so when the HP and $ numbers used aren’t right.