Step 1: you know it's not going to be $10, because that would be too easy and then it wouldn't be a puzzle.

Step 2: Assume that the case were $10. The wording says "the camera costs $300 more than the case itself". So that would make the camera alone $310, so camera + case would be $320.

Step 3: Since that's clearly too high, your next bet would be to reduce the price of the case as MaHe pointed out. Plug in $5, now the camera becomes $5+$300=$305. Add the case and you've got your $310.

Step 4: The answer is how much change, so subtract that $5 from $100. Pretty easy overall.