Wouldn’t 13 gold coins have been a huge fortune at that time, the life savings of a merchant, and unlikely to have been dropped accidentally, or even carried out of the house?
I wouldn’t say a huge fortune. It’s right at 19 ounces of gold.
For example, a standard British soldier would have made six pence a day (he did, however have to pay for almost everything). That would equate out to a gross pay of just over 9 pounds sterling. The average ratio of gold to silver was about 1:16 at the time, so 9 pounds of silver would work out to 9 ounces of gold.
19 ounces would have been a few years of gross income for a common soldier (and quite a find to be sure!), but to an officer or a rent collecting noble, it would have been maybe half a year of net income.
For someone like Mr. Bingley from Pride and Prejudice (who was stated as having income of 4,000 to 5,000 pounds a year), it would have been a fairly inconsequential amount. (4,000 pounds sterling would have been about 250 pounds of gold a year)