Speaking as a British person (i.e. not used to tipping), I'd say 5c a day.
Speaking for the instructors, I'd say $1,000 an hour.
In all honesty, what I've done in the past is discuss it with the group I've been in, and we will all throw in an amount. I'd suggest you go for more than 5c a day, and less than $1,000 an hour.