It really depends what kind of user you have and how complex the calculation is.
If you have a big pivot whith complex measures and your input changes all of that, the proposal above is the best solution. Disadvantage of that is your user needs to open the PowerPivot Window after he changed the input cell and refresh the Pivot after that. If you create a linked table with a column Sales_Factor your calculated measure is a simple multiplication of your sales measure and the Min, Max or average of your column
If your table is more simple and has a fix structure like ASP, volume, Sales, product cost & margin I would propose to transform your pivot into cubeformulas (options->OLAP Tools->convert to formulas). This converts every cell of your pivot into a formula that you can change afterwards. You can use the results of the PowerPivot calculation and add some additional input cells to modify the final results.