1. Keep talking about it with her. If she or yourself starts to get angry or raises their voice, start over later.
2. Throw logic right out the G.D. window when dealing with a mother who wants another. (I did the spreadsheet argument once, when she got done with me Satan was telling his buddies 'watch out for that guy, he's an a-hole')
3. If you bring up the word 'responsible' when arguing your side, watch out. You're talking financials, I know. That isn't what she hears, though. You're challenging her, in her opinion.
4. Try talking long term goals with her. See if either of your views have changed recently. If either of yours have, try to discuss why that happened, and whether you need to re-prioritize.
5. Always let her talk first. She gets emotional, and you might actually find out something totally not related to your immediate family that is causing an issue. For instance, a backstabbing sister in law that gets her kicks from stirring up chit just before holiday gatherings to make her effed up family seem better. Oops, sorry. Back to your situation.
This last one won't help your current pickle.
6. Never leave. Not angry, and not because she told you to. Let her know you're committed. Don't let her lock a bedroom door. Let nothing come between you. Doors are cheap, don't be afraid to break one. (something my youngest found out two years ago during a 'running away from home' hissy fit.