The planned cost is what you budgeted for an expense. So if I budget $5,000 for painting, that is my planned cost. If I actually spend $4,500, I have a $500 cost variance. Mathematically it is impossible to derive the exact planned cost unless I know both the actual cost AND the cost variance. If I know only the actual cost, I can use curve fitting techniques and show the planned costs and cost variance possiblities.