During adult stage at least, you can get away with
1 Gysahl Green per day and the chocobo will have full energy (minus what the Care Plan uses) the next day.
I think Azouph Greens are a waste of money if you're going the budget chocobo route. 1 Azouph Green = +17 Affection (a little more than 1/2 level of Affection) and 1 Gysahl Green = +11 Affection. You can feed your chocobo a maximum of 2 Azouph Greens (+34) or 3 Gysahl Greens (+33) per day. Much cheaper to just buy the gysahl greens probably.
Here's the research I've compiled so far.
Chocobo Food Hunger Satisfaction and Affection Chocobo Direct Care Affection
I've got more data on Direct Care and Care Plans, but I'm still compiling it.