To find the amount spent on marketing that will achieve the maximum profit, we need to analyze the quadratic profit function given:
\[ P = -4x^2 + 800x + 3000 \]
This is a quadratic equation of the form \( P = ax^2 + bx + c \), where \( a = -4 \), \( b = 800 \), and \( c = 3000 \). Since \( a \) is negative, the parabola opens downward, meaning the vertex of the parabola will give us the maximum profit.
The x-coordinate of the vertex for a quadratic equation is calculated using the formula:
\[ x = -\frac{b}{2a} \]
Substituting the values of \( a \) and \( b \):
\[ x = -\frac{800}{2 \times -4} = -\frac{800}{-8} = 100 \]
Thus, the amount spent on marketing to achieve maximum profit is:
\[ \boxed{100} \]
(meaning 100 thousand dollars).