Growing up, my Mom kept a notebook for this explicit purpose. Every monetary gift, given to family for birthdays, anniversaries, weddings, etc, she kept in a log. SHE expected the same thing back, same denomination, etc, when her turn came around.
When my cousins had birthdays, my parents would send the kid a $50 check. When my birthday/sister's birthday came around, my aunt & uncle would send a $50 savings bond. This pissed my mother off, b/c she couldn't immediately get the $50, had to wait the 20 years for it, and she knew they had only spent $25 on it. FYI - my parents kept all the bonds as their money, since they had laid out the cash for my cousins.
I had a destination wedding (Vegas) to keep costs down and the event small. My brother in law's best friend just so happened to be in Vegas visiting his sister when we got married, so of course we extended an invitation for him & his wife, as we were friendly with them. A week before the wedding, they guilted us into letting the sister come along too, b/c they felt bad about leaving her home alone. All three showed up to the wedding, had a great time, and the best friend handed my husband an empty card. I thought it was in poor taste, and I was particularly surprised because my husband had gone to their wedding and given a nice gift, not to mention that the wife was the type to be very into etiquette.
A few months later, we all happened to be hanging out together, and his wife got a little tipsy and started telling me stories about several people who attended her wedding and didn't give a gift! I told her that I had the same experience with a guest at my wedding, and all of a sudden, I figured out what happened. She had put cash in the envelope, and her hubby swiped it! He had married "up", and was having a hard time keeping up, so he knew we wouldn't say a word, and he took advantage of the situation. FYI - they didn't stay married long. I never told his ex-wife what happened, and I was so over it shortly after it happened. I was more annoyed that the hubby would do something so selfish and take advantage, after bringing extra guests! Rude...