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.