To find the amount of spending in thousands that will achieve the maximum profit, we need to determine the vertex of the parabola described by the profit function \( P = -5x^2 + 1000x + 5000 \).
The vertex \( x \) can be found using the formula:
\[ x = -\frac{b}{2a} \]
where \( a \) and \( b \) are the coefficients from the quadratic equation \( P = ax^2 + bx + c \).
From the given profit function:
- \( a = -5 \)
- \( b = 1000 \)
Now we can plug these values into the vertex formula:
\[ x = -\frac{1000}{2(-5)} = -\frac{1000}{-10} = 100 \]
Thus, the amount that should be spent on marketing to achieve the maximum profit is \( \boxed{100} \) (in thousands of dollars).