A realtor who is active in your neighborhood will be able to help you determine if building a new home would be a wise investment, or not, based on the other homes around yours. They will also be able to give you some information that will give you an idea of how much you could wisely invest into your home should you decide to remodel it.
In addition, a good contractor can also help you to review your options. Since you are trying to decide between remodeling and building it will help if you talk to contractors who do both, of which I am one and would be happy to help. When it comes to contractors, the best advice is to always meet with at least three. You will get different perspectives and ideas and will also be able to choose the one that best fits your needs.
Do a search on HUD's site and make sure to click the 203K box at bottom:
From a builders standpoint, and depending on your budget, you might just want to tear it down to the subfloor and rebuild from there.
A good amount of costs comes from the foundation .