Doing a little looking on the net I found that it used to be 21 shillings to a Guinea. There's 20 shillings to a Pound Sterling. So 30 Guineas equals 630 shillings, divided by 20 shillings/£ = £31.5 . I threw that in an inflation calculator and it claims that rifle costs £ 3798.36 today.
That's what I got, anyway.