I don't have a lot of experience in this problem but I would suggest that you be honest about it...both with them and yourself. Imagine if you ask for the $400.00 (or $800.00) and they say exactly what citahellion says, "Sorry we can't afford that.". Would you be ok with that? If so then you could say to them, "I'm sorry but I really can't consider this position for anything less than X".
If you are not willing to lose out on the position and can exist with the salary then you need to say something like this to them "This is less than I was hoping the position would pay. I am really excited about the idea of working for your company but this is going to make it difficult for me to achieve my goals financially. I know that people with my experience for this kind of position can make X, so this is something I am going to have to consider carefully."
This gives them an oppurtunity to make the decision easier for you. Even if they can't (and they may not be able to) it lets them know that you think you are worth more but are willing to work for them for less. If they want to hire you they will want you to be comfortable with your salary because they don't want good employees leaving. They also want to pay you the least amount they can.
Again, I think it works better as a negotation and not a demand...so I would probably go the second route regardless. Just in the first case if they don't come back then you tell that you would like to work for them but the salary offered is too low. On the second you will probably take the job either way but you are not risking as much by trying to get more. I doubt they will be offended that you need more money...everybody needs more money.
Personally, I have not had a job so great that after a while more salary wouldn't make it better. I also think there is no job so great that if it doesn't meet your financial needs that you HAVE to take it. No matter how happy you are with your job if you are struggling financially you are going to have problems. I bet you would eventually take another job that pays better anyway. Better to find a job that satisifies both needs as equally as you can.