Two different fitness clubs offer membership plans. Club A charges a one-time enrollment fee plus a monthly fee. Club B charges a different one-time enrollment fee plus a different monthly fee. The graph above shows the total cost, in dollars, for $x$ months of membership for both clubs. If the line representing Club A has a $y$-intercept of $50$ and a slope of $25$, and the line representing Club B has a $y$-intercept of $70$ and a slope of $20$, what is the difference in their monthly fees?