The Camera and The Case

The hints explain this puzzle fairly well

While the puzzle traps many players into initially thinking that the camera costs $300 and the case $10, this doesn't satisfy the condition that the difference in price is $300

As the total price is $310 and the camera must cost over $300 (being the cost of the case plus $300), there's not many possibilities to consider

You can also consider that the cost of the case is the same as the cost of the camera less $300 and that twice this value must add up to $10

From here, it should be fairly easy to work out the cost of the case and how much change you would get from $100