By Box turtles you mean the genus Terrapene then they all stay more or less the same size with some populations within each species being smaller than other, Gulf Coast tend to get the largest. About a pound.
Affordability as a questions raises the eye brows of anyone who might answer your question. There is no such thing as a cheap turtle. For the most part over the life of the turtle the difference in cost is negligible when you consider materials and effort to care for it.
If you have a secure outside enclosure in a range area of the species you get, long term care is sorta nothing, in which case why worry about the up front cost. If you are going to have the turtle as an indoor pet then those maintenance costs will outpace the animal cost pretty quick. Without a better understanding of the concern for cost, it really doesn't matter.
If you can apply some patience you might be able to get one from an animal rescue, or local T&T society at no cost or a low donation or administrative fee.
How old are you? It always help to understand about offer a point of view if I can know who my audience is.