As you enter the ballroom, the cards box and the gifts table will be way in the back, by the large windows, away from the entrance, for obvious security reasons.

How much to give? Obviously they want to see you more than your money, but still, weddings are expensive.
The couple is probably spending $125 - $200 per person for their wedding, so, if you can, consider covering your own cost so the couple does not start their new life in a hole.