If your number is below theirs, you potentially just cost yourself a chunk of salary. If you aim too high to avoid this outcome, you could look delusional.
That's why it is recommended to thoroughly research and understand your market value in the location and field that you are negotiating. Talk to other developers in the area, research online, etc. - finding out reasonable salary expectations is not impossible.
I typically reply with, "quarter-mil." Then wait. Typically they will respond with something that hints to their budget so I can give a more informed number. My initial number is high enough that they know I'm not being serious, but opens up the number discussion.