I thought copyscape can be used for free.
If I am not wrong then you can check 10 "links" per month for plagiarism checking for free.
I have quoted the word link because your content has to be posted in your blog/site and then you can check for plagiarism.
In my case, the articles I have are in plain text. So plain text check needs a premium search and it's not free.
Still looking...
If you hesitate to use your PayPal/card in my account then you can create a new Copyscape account on your own and then make the deposit.