Need some advice concerning ethics...

Is it unethical to back out on a job that you accept but then find out that there either isn't A) any affordable housing (even considered paying up to 30% of your income which is high in my opinion) and B) any housing that you do find affordable isn't located in a safe neighborhood (which, in my opinion, defeats the point). Ok... that is a bit of complicated lead off sentence... sorry.
I just can't rationalize taking a job where I am going to bankrupt myself staying in a motel waiting for an apartment to open up but I also don't want to come off like an ass to this company.
Any thoughts or suggestions?

So basically you're asking if it's unethical to commit financial suicide?

In a perfect world, you wouldn't walk away after saying yes. However, in your case I'd just tell them that I have to leave because the salary isn't enough to live within a reasonable commute of the job. At that point, the ball is in their court. They know how to reach you if they decide to make a better offer.