" She followed another carriage to Mr. Cole's door; and was pleased to see that it was Mr. Knightley's; for Mr. Knightley keeping no horses, having little spare money and a great deal of health, activity, and independence, was too apt, in Emma's opinion, to get about as he could, and not use his carriage so often as became the owner of Donwell Abbey."

I think it is clear from this that Mr Knightley prefers to spend his money on paths and crops, and his grazing land on wool and milk. He can get to London, Kingston, faster and cheaper on his horse.
Contrast with Mr Elton who, as a single man, was also going to London on horseback, and who catches the stage coach to Bath. For him, love and marriage go together like horse and carriage - that he could not afford before he married, and that he possibly can't really afford to keep after, but which is an absolute necessity for the Eltons if they are to assert their place as Lord and Lady Patroness, the first couple of society in Highbury.

I am guessing that, like Mr Martin, most of Mr Knightley's money has been put into capital- "The plan of a drain, the change of a fence, the felling of a tree, and the destination of every acre for wheat, turnips, or spring corn" rather than squandered on a couple of beasts kept purely for idle show. He can afford to hire horses whenever he feels the need to take out his carriage, and can get where he wants to easier without the encumbrance. He truly is the Squire and the Patron of the parish, and Mr Elton seems to have the job of spending other people's money: on Parish, Mr Knightley, Mr Cole and Mr Weston's money, and in private,the late Mr Hawkins. Mr Knightley has nothing to prove and "No taste for finery and parade".

I am guessing that he is, in spite of the lack of horses, a much wealthier match than Mr Elton, by the reaction in the vicarage. Mr Elton's assumption that "she had always meant to catch Knightley if she could;" and his wife's "How could he be so taken in? Did not think him at all in love -- not in the least." exactly match their own approach to marriage, and I can't imagine such a perfectly matched couple being so discontented by the blow, if Poor Knightley had thrown himself away on Emma's £30,000. And they married in October, just after Michealmas, when the annual harvest and the rents come in.