The answer is L. The sum of 3rd column minus 2nd column and 3rd column minus 4th column are equal (without third row). The answer should be the average of 1(2nd column 3rd row) and 23(4th column 3rd row) which is 12. Latin 12th letter is l(L). The answer is in lowercase since all other lined squares have lowercase letters. Used same valuations with BudBudderly from highiqforum.

As second guess because currency spelling is read in lined squares. The letter L symbolizes Litecoin, which is Ł.