I'm fostering a dwarf lop at the moment. She is over 3kg (we are trying to get her weight down though). Just like Sammi_83's bunny, she eats a lot of hay. I buy hay in a bale, which costs about £8 and will last a few weeks for 2 bunnies. Bunnies aren't usually meant to eat too much of the processed bunny food (the pellets), so just like Sammi I only give a few of these a day, so that doesn't cost much either. Then some fresh veg, or in the summer I pick dandelions and brambles and things instead, so they are free!
What is the costly bit are the vet's bills, though :lol:.