What's £6 then buy today? Reads like £6 30s 4d is the exact fractional amount?
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.
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