If you own a stable of a bikes and like to feel like you are making the best of a situation, buy double-ended road/MTB brake cables. When you go to replace an existing cable, cut the new cable only as long as needed to replace the original. The other portion can then generally be used as well, either for a front brake cable if you used the rest for a rear or vice versa. This works on my medium sized bikes but might not on anything larger.