How often? Once a day
What do you buy? Produce: bananas, mangoes, pears, kiwi, strwaberries, blueberries, avocado, greens like spinach, kale, chard, romaine, tomatoes, cantelope, etc.
How much do you usually spend? $15-20/day
Do I think too expensive? No, the way I eat I avoid medical costs and am happy and fit.
Do I think it would be easy or difficult for a family near poverty line to go vegan? If their hearts were in it, yes, very easy. Veganism can be done if your ethics dictate that you don't want to cause harm to animals. There are many cheap bulk, dried (inferior nutritionally but doable) items a person could buy.