How much to give to memorial fund?

We typically make a contribution equal to about the amount we would have spent for a nice floral arrangement. That can be anywhere from $25 up to $100. from there let your heart and your financial situation guide you. many times the recipient's family is notified simply that "...a generous donation has been sent to _________ (charity, church, etc) by Mr. and Mrs. Joe Jones." when we've received those notes, no amounts are generally noted, just that a donation has been made and by whom.
Most will prefer a contribution vs. flowers as the flowers wilt and die in a week, but the donation to a church, cancer foundations, hospice care facilities, etc., go to do real good in the name of the person being remembered.

and don't be afraid to ask the family if they would prefer flowers or a contribution and if it's a contribution, to which organization.

If it's a close friend and they say absolutely nothing, give them a tree to plant in their yard, an evergreen perhaps that they can decorate at the holidays.