Let $m$ = male, $f$ = female. Constraints:
$40m + 50f = 1000$, $f>7$, $f \leq 12$. Cost = $250m + 15(40m) + 300f + 10(50f) = 850m + 800f$. Minimize cost → minimize $m$ for given constraints. From equation: $m = \frac{1000 - 50f}{40}$. For $f=8$, $m=10$. Cost check shows minimum.
\[
\boxed{10}
\]