Very affordable.
But first, let me say it does depend on what kind of vegan you choose to be. If you choose to buy lots of plant based meats, milks , fast food, restaurant food, and prepared and/or frozen foods - it will be less affordable because most of those things are not cheap and for the most part they are more expensive that their animal based counter parts.
I used to have a link that had a chart of the price of a gram of protein in things like eggs, milk, meat, beans, etc. and now I can't find it. but... as I remember it eggs, cow's milk, and chicken are some of the cheapest sources of animal based proteins. but they aren't nearly as cheap as things like rice and beans.