There is no fixed amount. An amount between $10-20 are acceptable, depending on how close you are with the family. The tributes are typically placed in an envelope and left with a family member of the deceased. The tributes are typically used by the family to cover funeral expenses which can cost from $5,000.
More families today are donating the tributes to charities in memory of their loved ones - a growing trend of "legacy giving". Visitors are also more open in giving more ($50-$100) when they know that the condolence money is channeled to charities.