I am actually in the exact same situation, except i moved home because she got laid off. My cure: visine in her coffee. It gives her the shits, wont kill her, and any juvenile act towards an equally juvenile person always seems to make me feel better. Its very bad advice, but I'm quite literally fighting for my own sanity 90% of the time. Another thing that has helped is seeing how truely unhappy she is, and in knowing what makes her the way she is, in understanding she'll NEVER change, I've come to see no matter what she says or does will ever change the fct that she is still my mom. Neither one of us really have anyone close outside our small family of women ( her, myself, and my two sisters). Another is simply keeping myself busy and out of the house. To each his own. I hope maybe my own miserable experience has in some way helped.
-allison

P.S. About the bill: pay them what you think is fair. Because they won't tell you how much they want, they are leaving it up to you in a way. Take initiative. Then, when they bitch that its not enough, ask them to see the bill. Its only fair if you have to pay a portion.