Mr. Dashwood could reasonably have saved that amount if he had had that income for 19 years--but he didn't. The Norland estate earned that amount, but Henry Dashwood only had that income for himself for one year. Before that, the income was his uncle's, to dispose of or distribute as he wished.

The £10,000 included the £3000 left to the girls by their father's uncle and £7000 which Henry Dashwood had to leave to his wife. We don't know if that was money he had managed to save up over the years, or if that was his own from before. Mrs. Dashwood herself brought no money to their marriage.