When using a flexible budget a decrease in activity within the relevant range does what?

I wanted to get this answered more fully, and correctly. Decreasing variable costs per unit is just wrong.

When speaking of variable vs fixed costs, it means in total. A variable cost stays the same per unit, but as volume changes, the total variable costs increase and decrease. (Unless something specifically mentions there's a change per unit.) A fixed cost is fixed in total regardless of volume. But fixed per unit increases and decreases with volume changes. In order for variable and fixed to have their proper meanings, you have to think about them as total costs.

For example, if I buy a certain shirt for $7 and sell it for $15, those are variable. They stay the same per unit and I gross $8 per shirt (called contribution margin). The more I sell, the more sales revenue I have and the more variable cost I have -- two shirts will have $7x2 ($14) of variable costs etc. If my fixed costs are $100,000, that will remain fixed regardless of how many of anything I sell. An example of a fixed cost is rent.

If activity decreases, total variable costs will decrease, but not per unit variable costs. Total costs also decrease, but that's not complete. And fixed per unit increases, because you don't have as much volume to spread the fixed costs over.