Let the cost of a 2 inch marble be \( x \) cents. Therefore, the cost of a 1 inch marble would be \( x - 30 \) cents.
Rochelle buys 25 1 inch marbles, and the total cost for these marbles is given as $10, which is equivalent to 1000 cents. Thus, we can set up the following equation:
\[ 25 \times (x - 30) = 1000 \]
Now, simplify and solve for \( x \):
\[ 25(x - 30) = 1000 \]
Dividing both sides by 25:
\[ x - 30 = 40 \]
Adding 30 to both sides gives:
\[ x = 70 \]
Therefore, the cost of a 2 inch marble is \( 70 \) cents or \( 0.70 \) dollars.
Thus, the answer is:
\[ \boxed{0.70} \]