I agree that the old uncle thought he was being kind by leaving such a gift to them. As mentioned in other posts, he had no way of knowing that his nephew would die so soon after he did.

The Norland estate had an income of 4000 pounds a year, which is very sizeable. Had Henry Dashwood lived longer, he probably would have been able to enlarge his girls' dowries. There is no indication that they lived extravagantly. The only indication that perhaps Mrs. Dashwood had not a lot of money sense was this from the start of Chapter 3:

[Mrs. Dashwood] was impatient to be gone, and indefatigable in her inquiries for a suitable dwelling in the neighbourhood of Norland; for to remove far from that beloved spot was impossible. But she could hear of no situation that at once answered her notions of comfort and ease, and suited the prudence of her eldest daughter, whose steadier judgment rejected several houses as too large for their income, which her mother would have approved.

I always find myself wondering if, during the 10 years that they stayed at Norland and looked after their uncle, whether they had any real income of their own. It doesn't seem possible that Mr. Dashwood could have done much in the way of a dowry for his daughters before he inherited.