Sometimes, construction employers are looking for your acceptable construction salary range in order to screen you out if your number is too high. Sometimes, they are looking for a beginning point for your offer. Either way, it is just fine to answer the question with something vague like "negotiable." If your qualifications are strong, they are unlikely to wash you out because you didn't give a number. On the other hand, if your qualifications are weak and you want the job, a low number might improve your chances.