I can't give you a dollar amount. I've created my own FE gifts in the past. Things I like to make and give.
Some FE groups set a dollar limit (minimum or maximum).
For me, $10 per person is just too much, but I've been involved in some fairly large FE groups.
Fish Extender gifts were started as a fun thing to enhance people's cruises. I'm saddened by the change to the one ups-man-ship involved now. Gifts used to be given anonymously, but now, many groups require names or some other identity info so you can figure out who gave the "best" gift.