I tend to just translate a few key numbers to get a rough conversion factor. From S&S, we are told that 1000 pounds per annum is about the minimum income for a middle class gentleman with a wife and family. The modern equivalent income is perhaps 60,000 (British pounds, US, Canadian, Australian or New Zealand dollars - curiously the value of a unit of currency correlates reasonably well with living costs.) So my rough conversion factor is 60. As a check - when Catherine Morland went to Bath, her father gave her ten guineas* spending money - about (currency-of-your-choice)630 - seems reasonable.

However, from other links in this thread we see that a labourer got only about 50 pounds per annum. Modern equivalent would be about 20,000, for a conversion factor of 400.

So if Elinor spent 2s (0.1 pound) on a cab fare, it would be like she was spending (currency-of-your-choice)6, but for her maid it would be like spending (currency-of-your-choice)40.

* A guinea was 21s, as opposed to 20s in a pound. Don't ask me, I didn't do it.(And if any American wants to complain, I have a few things to say to you about feet, miles, pounds, ounces...)