Ok, so you get a card and $100 USD cash from that former boss of your father whom you met when you were a kid, nice you might think, but it doesn’t really end there. To thank the person for the gift, you need to thank him/her with a gift of your own. This time cash is no good; it should be a physical gift and the general guideline is that the list price of the gift should be in the vicinity of 50% of the cash you received in the first place. Here you have a minor opportunity to save some money in case you can find something at a discount, the gift usually is some form of plate, glass or similar thing.
So now you got the cash gift and you spent basically half of it on getting a thank you gift to send back to the person and he/she in turn will send you a thank you card thanking you for the gift. Depending on the nature of the new thank you card you might need to consider sending a thank you postcard back to the person and pray that it ends there.
Broken down, the flow looks something like this, 4 active steps:
$100 USD cash gift received - $50 USD spend to purchase and send a physical gift back – Thank you card for the thank you gift is received – Thank you postcard or e-mail is sent to thank the person for the thank you card – END
I personally feel that if the flow could be simplified into these two active steps:
$50 USD cash gift received – Thank you card sent – END
...everyone would be happier in the end and save us all a lot of trouble and time, however keeping things simple is not always how the Japanese like to keep it...