When sales and costs change at the same time, start by isolating one change and add the others afterwards. This avoids blaming the whole change on the most eye-catching factor.
An example with numbers
You sell 100 units at $40 with a $25 variable cost. Contribution is $1,500. If the cost rises to $27, it falls to $1,300. If you also sell only 90 units, it ends at $1,170. In that order, $200 of the fall comes from cost and $130 from volume.
Try it with your numbers
Build the three scenarios with the same fixed costs. Note the order you used: when factors interact, how the difference is split depends on that order.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.