I have passed the buck, which may not surprise anyone, and applied to the L&T archives and the estimable JulieW to produce an answer. I have linked Julie’s post about whether JA speaks of gross or net incomes using Mr. Bennet and Longbourn of P&P for example. (:D)

Hot potatoes! LOL! It also may be that Willoughby has at least one hunter in addition to the four horses you suggest. Sir John remembered that Willoughby “at a little hop at the Park, danced from eight o'clock till four, without once sitting down” and was then “up again at eight to ride to covert” per Ch. 9. A definition at dictionary.com of covert is “Hunting. a thicket giving shelter to wild animals or game.”

Selling the horses illustrates how Mrs. Dashwood, probably at Elinor’s insistence, has retrenched in order to live compatibly if not so elegantly with their reduced income. Is there reason Willoughby cannot drop his hot potatoes and gain the upper hand over his financial difficulties? I think it must be an unwillingness to sacrifice for it seems he purposely lives “at an expense to which that [six or seven hundred pounds] income could hardly be equal” per Elinor in Ch. 14.