i generally will leave a $1.25 tip for breakfast if eating alone in these parts. since an average breakfast tab here is $5 or so, it's a generous 20%. life is fairly cheap here, so it goes farther than you think.
on a comp Hilton breakfast, i've traditionally left $2-$3. and yeah, i do love how it is in France; i always am very amused when the table-full of 'merrikuns leaves 20% over the bill. i got to explain that to a very confused French waitress once. given how boorish the group was, though, i think she almost deserved hazard pay.