Well, ten thousands in a commoner are a moderate fortune compared to Emma's thirty thousands and status : Emma was far a better "catch" to Mr Elton (besides all her superiorities, that Mr Elton could not feel); and it was lower than the twenty thousands per girl in the family he had had a hint of to Mr Knightley.
But it was all cash, like Miss King in Pride and Prejudice, who was briefly engaged to Mr Wickham. It was in fact a big dowry, even if nothing was to be expected after, but dowry hunters expected not less... They wanted to make their own fortune by marriage, which was not the general scheme in XVIIIth and XIXth centuries England.
As for Emma, her way of living is already very modest for the co-heiress of a family estate of a total value of sixty thousand pounds; it would have really been totally out of normality if she had already had thirty thousand pounds and could expect more. In addition, Jane Austen's expression is well that of the total amount of her share of the family fortune, present and future; all what could be expected at her father's dead, is necessarily included. What we do not know is the proportion, in this total of thirty thousand pounds, what she already owns, what she will on at her marriage, and what she will inherit from her father. To me, she has probably already or will have at her marriage, five or ten thousand pounds. The left is her father's fortune, of which she is co-heiress.