[blintzing]

https://en.wikipedia.org/wiki/%C2%A3sd

The Latin librae, solidi, and denarii became pounds, shillings and pence.

There are 12 pence in a shilling, and 20 shillings in a pound.

So, "£6 30s 4d" is (6)+(30/20)+(4/240)= £ 7.51

It's rather odd that such an exact amount was given, which is what the sentence points out.

[jakeogh]

Found a calculator! http://www.measuringworth.com/ukcompare/

In 2014, the relative worth of £7 10s 4d (that's what it converted to) from 1619 is:

  £1,320.00	 using the retail price index
  £1,300.00	 using the GDP deflator
  £52,300.00	 using the per capita GDP
  £400,000.00	 using the share of GDP