Let the cost of a 2-inch marble be \( x \) dollars. Then, the cost of a 1-inch marble is \( x - 0.30 \) dollars.
Rochelle buys 25 1-inch marbles for a total cost of $10. We can set up the equation based on this information:
\[ 25 \times (x - 0.30) = 10 \]
Now, simplify the equation:
\[ 25x - 25 \times 0.30 = 10 \]
\[ 25x - 7.50 = 10 \]
Add 7.50 to both sides:
\[ 25x = 10 + 7.50 \]
\[ 25x = 17.50 \]
Now, divide both sides by 25 to find \( x \):
\[ x = \frac{17.50}{25} = 0.70 \]
Thus, the cost of a 2-inch marble is \( \boxed{0.70} \) dollars.