It depends on what you mean. If you mean that it cost less yesterday or last year or that it will cost less in the future, then the sentence is correct. This is because the verb ‘cost’ is used in the past tense and the future tense. However, if you mean that it costs less today, then you need to add an ‘s’. It probably would enhance the sense of the sentence if you added some type of time modifier. For instance, “It cost less one hundred years ago to reuse boxes than to buy new ones.” This way, the reader clearly understands you mean the past tense. Or you could say, “It costs less today to reuse boxes than to buy new ones.” Again, the reader understands that you know your verb tenses and that you are talking about the present, here and now.