S.B.
I always thought that thank you cards go to people who went above and beyond visitatioin and cards being sent. Those who sent flowers, money, pall bearers, friends who helped (watch kids, transportation, provide meals, etc.), the clergy , that sort of thing.
While writing notes to everyone is a wonderful sentiment, it seems like an incredibly arduous task and inevitably, someone will be missed.
I think sending money is common practice. Usually it is intended to help with funeral costs. It can also be donated to a cause important to the family. When my husband's grandfather passed away, all money donated was pass on to American Heart Association.