defects of weight by cost

When we use cost as the standard of split weight in progress control, we found that some work in construction stage spend several days, but it take big part in weight because of cost. When the material procured, we take some actural percentage in progress. But I think it is still difficult to correctly present the progress of construction works.

Anyone knows how to make it better for progress control when we use cost as the weight?

I hope you can share your experience in progress control with me.

Thanks!