First, income reduction is there to delay teching, not to show the power of a unit.
Then how do you balance a unit/faction if said unit is harder to get? There are effectively less units of that type on the field. It must be somewhat stronger.
Or if the developers wanted there to be less vehicles with the same power as your opponents vehicles, OKW would have to have an (intended) advantage in infantry to balance the faction.
Not defending the current performance of the units, but I think the fuel argument isn´t good.
Second, even if we took the income reduction in the price, it would be 150 fuel for the Stuka (100 x 1.5).
Ha, I didn´t have maths since six years. I never liked it.
But that makes the fuel difference even higher.
Add to that no caches and you can figure out that OKW is currently designed around having less but more effective units on the field. Thus a higher survivability on the Stuka can be explained.