In my opinion, the whole fortune of the Woodhouses is 60.000£, inclusive of Hartfield, maybe a bit more if Isabella is supposed to get more (but it is not certain).
It is already much more than the average gentry family who had about 700£ a year, that is, a fortune of about 14.000£ (per whole family).
Jane Austen writes that Emma is "the heiress of thirty thousand pounds".
Would she have already inherited them, from her mother, would have meant that the late Mrs Woodhouse whould have had a fortune of thirty thousand pounds, that is, twice that of Pride and Prejudice's Miss Darcy's, who is much upper (her mother was the daughter of an earl, who seems to have married considering her husband's huge estate) then she would probably have an establishment in town (thus Isabella would have moved in when marrying Mr John Knightley, instead of buying or renting a new home, as Brunswick Square houses started being built only in 1795). And Jane Austen would have written "the owner of thirty thousand pounds" or "the heiress of seventy thousand pounds".
And at Hartfield, she would have probably led a brighter life.
Already, I have written elsewhere that they were living with less than half the income of a sixty thousand pounds family fortune.
My opinion upon the late Mrs Woodhouse's own fortune, is that she may have brought up to twenty thousand pounds (which was a lot), and that this part may have already belonged to her daughter, or half of it, probably shared equally.
That is, it is not unthinkable that Emma (and Isabella) had already five or ten thousand pounds, and the left, would be at the death of their father.
As to Isabella inheriting Hartfield, it was the normal scheme as she was the eldest daughter and while Emma had not married Mr Knightley.
But Emma marrying the owner of the neighbouring estate, I think it could have been considered too, that Hartfield may merge with Donwell to create a larger estate; Isabella then, getting money or shares and Emma, Hartfield. Anyway money was unlikely to lack, as the Woodhouse's fortune was not mainly of landed estate, and as they were far from spending their income.