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Since cost of vehicles (preasumably) were based on their Survivability+Firepower+Special abilities, their costs should be reconsidered after 6ed introduced drastic changes in Survivability, that screwed one vehicles and elevated other (firepower is harder to grasp in general).
My proposition:
Vehicles with 3HP - no cost change;
Vehicles with 2HP - cost -20% (/1.2);
Vehicles with 4HP - cost +20% (*1.2);

First: Doesn't belong in YMDC.
Second: HP change is peanuts as far as game efficiency is concerned. If you want to rebalance the game, start by recosting all Flyers (huge bonuses for minimal penalties and no point increase) and Skimmers (Free smoke launchers without penalty if you move. Compare to 5e's flat-out cover save that mean you could not fire.)

Calculating 20% decrease is not done by dividing with 1.2

It should be Cost*0.8

You could also comment on , you know,
The Idea
and not just the math thing.

Ok, it's a terrible idea.

Vehicles are either fine or they aren't. Just like anything else. They don't get to be recosted because of edition changes.

You kinda ignored the whole thing about vehicles getting cover saves easier now. And that they aren't stunned/shaken as much now. Survivability is less in some instances. Firepower, however, has increased.