What information you have looked at? - The advertised rate for 2nd Class was £13 per adult passenger, except for the outside rooms on D-deck where the rate was £13.10.0. - A number of 2nd Class passengers paid less than the advertised rate, because they had originally booked to sail on other ships.
The Minimum 1st Class fare was £26. That could be reduced to £23, if one booked not to take meals in the Dining Saloon. - The advertised rate for the inside single-berth rooms on E-deck that you refer to was £30. - While many 1st Class passengers paid less than the advertised rate, none [except those who held complimentary tickets] that I am aware of paid less than £23.
Are you perhaps looking at the overall cost of the tickets and not allowing for the number of passengers who travelled on a particular ticket?
Bob Godfrey once told me that an easy way to convert 1912 pounds in today US dollars is adding two noughts to the original number. For example, a 2nd class cabin costing £13 is more or less $1300 in US dollars nowadays.
P.S. Please note that this way is easier but not the more accurate.