"I had much rather go in the coach."

"But, my dear, your father cannot spare the horses, I am sure. They are wanted in the farm, Mr. Bennett, are not they?"

"They are wanted in the farm much oftener than I can get them."

"But if you have got them to-day," said Elizabeth, "my mother's purpose will be answered."

I can see your point, Lizzy could be poking fun, although I can't see anything in the text to confirm this - she certainly doesn't disagree with the scheme of her mother's, and her remarks to her father as above suggest his compliance perhaps?
But as I say, in P&P2 she isn't agreeing even in jest, she's horrified!