Actually there is a specific answer to this question in the book.  It is in chapter 3 when the narrator who we later learn is Crispin is giving us his history.  When at first, Crispin is telling us what a difficult life he and his mother led, he tells that his mother's wages were.  Hers were a penny a day and she could buy full loaves of bread on this.  So specifically. a quarter of a penny would be worth a loaf of bread.

It is one fourth of a penny. Money back then had much more value than our money does today. For us, the idea of one fourth of a cent is ridiculous! There isn't even anything that costs a penny anymore. A penny back then was more like our dollar today or even higher. A copper then was what our penny is like today. They had half penny and quarter penny coins as well. There was also a silver penny as well as shillings and guineas. Many of the portion of a penny coins were phased out after the economy grew with skilled labor. I'm sure a quarter penny might buy a loaf of bread or a mug of ale in those days.