Esakkimuthu Hi,
Yes, MOST of the plagiarism detector are not free.
But you, through your institute can ask for the trial version of the same, that will help others also, in the institutes. Subsequently it may be purchased. Because, now each and every institute must take care of “Plagiarism” considered as academic theft. UGC(India) proposed regulations for the awarding of the degree in India that all students must begin using well developed software to detect plagiarism and other form of academic theft. So, university ask for plagiarism report along with the submission. A plagiarism check report is a mandatory requirement. Universities may cause penalties, suspension, and even expulsion.
Some of the FREE version you can find on the net but they are not useful, because of the limitation imposed on the same.
Regards.