If 2,100 units were sold, what is the sales price variance?

The actual sales revenue was \10,080$ for 2,100 units sold.

$$\text{Actual Selling Price per Unit} = \frac{\text{Actual Sales Revenue}}{\text{Actual Units Sold}}$$

$\text{Actual Selling Price per Unit} = \frac{\$10,080}{2,100 \text{ units}}$ $\text{Actual Selling Price per Unit} = \$4.80 \text{ per unit}$

The budgeted selling price per unit was \5$.

$$\text{Sales Price Variance} = (\text{Actual Selling Price per Unit} - \text{Budgeted Selling Price per Unit}) \times \text{Actual Units Sold}$$

$\text{Sales Price Variance} = (\$4.80 - \$5.00) \times 2,100 \text{ units}$ $\text{Sales Price Variance} = (-\$0.20) \times 2,100 \text{ units}$ $\text{Sales Price Variance} = -\$420$

Since the actual selling price per unit (\4.80$) is less than the budgeted selling price per unit ($$5.00$), the variance is adverse.

The sales price variance is \420$ adverse. This corresponds to option C.

The final answer is \boxed{\text{C } \420 adverse}$.