ויקרא פרק כז, יג וְאִם- לַע ֹּותִ שיִמֲח ףַסָיְו הָ נֶלָאְגִי לֹּאָ
ג- : ָךֶ כְרֶע תורת כהנים זה בבעלים אמור
If the owner redeems the animal from Hekdesh, then needs to add a fifth (which means a fifth of the total sum after the addition value is added-in plain words one adds 25%, making the added value 1/5 of the total result (125% / 25%)). This applies only to the owner, since the verse uses the word גאל, which is applicable mainly to the former owner, since anyone else would be acquiring (and would have used a word such as קנה ).
It is proper for the former owner to add a fifth to the value since the object has more value to the former owner (sentimental or otherwise). Further, the mitzvah of redemption only applies to the former owner. Please note that Rambam at the end of Hilchos Terumah provides another reason.
