I think JA deliberately kept silent on exact figures regarding the income the Kellynch estate would have produced because at the time she was writing, the economy was in flux. Inflation was rising as was unemployment and after the end of the Napoleonic wars, the profits produced by agricultural land during war time were eroded. The good times were gone. If she had set a figure it might have looked dreadfully out of date by the time of publication.

Look at this from Janet Todd's introduction to the Cambridge Edition of Persuasion:

Austen's vision of prosperous Abbey Mill Farm in Emma epitomised the agricultural flourishing of landowners and richer tenants in the war's final years, when Napoleon's Continental System barred countries under French control from trading with England and when internal prices and rents were kept high. After 1815 as inflated prices fell rapidly, the British Goverment appeased worried landowners by bringing in the Corn law Act banning imports until the grain price reached£4 a quarter. This measure caused considerable hardship for the poor and made it clear to many that the landed gentry did not regard themselves as heading and defending communities but as looking after their class interset...Sir Walter's faliure in Pesruasion to manage his estates propely in the "good" years and his lack of concern for his tenants when he abandons Kellynch-hall need to be read against this background....

William Pitt the younger had rebuilt the financial foundations of Britain which then enabled him to subsidise European armies to fight France. He did so by running up a huge public debt to the level of £745 million and by bringing in an unpopular income or property tax. In 1816 this tax was abolished and consequently indirect taxation fell upon the midlding and lower ranks more heavily than on the very rich. Analysing the "very general depression' at the end of the Napoleonic War from the vantage of the 1830s ,Thomas Tooke noted losses and failures in agriculture commerce and manufacture which caused "extensive suffering and distress". Beyond the bad harvest and European poverty which prevented the expected buying of British goods, the depression was caused in his view by the "transition from war to peace'
pages xxxiii-xxxiv