Multivariate Linear Regression with exponential trend-line
I would suggest transforming &quot;qty&quot; into the log space. Even, you can do this using Excel. You can make a new column (for example qty_log), which is equal to the log of &quot;qty&quot;. Then, you can fit a linear regression as follows:
$cost = a1 * log(qty) + a_2 * volume + a_0$
You will get much better results in this case.
This equation simply means that try to fit a model using volume and log of qty for predicting cost. You did so before but you used qty instead of log(qty)