I think, appears at the end of Ch. 4.
Mrs. Dashwood is highly offended by Mrs. John Dashwood, just in time to receive the offer of an affordable rental a long ways away. Mrs. Dashwood makes up her mind immediately - "Her resolution was formed as she read" and she writes back directly, but then she "hastened to shew both letters to her daughters, that she might be secure of their approbation" before she posts the response.
However, despite her apparently disclosing her plan to at least both elder girls, only Elinor's reaction is noted in the text. She is, of course, strongly for it. Based on Marianne's highly dramatic sighing over Norland in Ch. 5, I have to suppose that her reaction to the idea of moving to Devonshire is less positive than Elinor's. But Elinor supporting the plan is enough to move it forward, and there is no indication of Mrs. Dashwood hesitating or reconsidering for the sake of M's feelings.
So, even in her naturaly romanticism and highly emotional state, it appears that Mrs. D is aware of the advisability of attending to Elinor's cousel on practical matters. She enters into Marianne's feelings, but does not appear to be steered by her financially or behaviorally. She may not have tremendous common sense herself, but she seems to know it when she hears it. I wonder if this is not another indication that perhaps Mr. Dashwood was a naturally prudent man, since Mrs. Dashwood appears to be in the habit of attending to prudent counsel.