Turbine Blade Construction

You've been studying "airfoils".

Weight. Inertia. Strength. Cost.

is weight the only consideration ?

No. Many other factors apply. (see above)

Overall lighter blades are easier to transport and install, give better response in aircraft applications and can be cheaper, too.

that makes sense , but wat about the strength of the blade

or has it already been accounted for ?

This subject is far too complex to discuss here. You need to do some basic research to understand the many, many factors involved in the design and manufacture of air foils, beams and turbine blades.

On a motor vehicle, the prop shaft driving the rear wheels is thin and hollow. This will handle more torque than a solid (and smaller diameter) shaft of the same weight. Now extend this argument to prop blades etc.