18"
There is seldom a cheap and easy way to do it. You will have to open the floor, easy or hard depending on what it is made of, then cut the pipe to the toilet and insert some fitting to deflect it to the new location, again the degree of difficulty depends on how it was originally installed and which way the pipe goes. 13" from the side wall was never a proper measurement anyway. It should have been at least 15" but sometimes it is easier to offset 5" than it is to do it 3".