Slope of the Budget Line
They do consider it. The precise mathematical formulation is usually
$$
-\frac{p_1}{p_2} = \text{MRS}(x_1,x_2)
$$
or
$$
\frac{p_1}{p_2} = \left| \text{MRS}(x_1,x_2) \right|
$$
in the basic cases, when MRS is a negative number (the slope of the indifference curve at basket $(x_1,x_2)$).