If you sign up for the insider program, you can download the Windows 10 insider preview for free, and later can download the full version for free.
If you "upgrade" from another version of windows, you can upgrade, license the product and then if you wish you can download an ISO of Windows 10 to perform a clean install at some later date. So long as you upgrade during the first year after Windows 10 is released, it will not expire and will always be free for you. If you don't sign up for the insider program NOW, or don't upgrade your copy of Windows 7, 8 or 8.1 during the first year after Windows 10 is released, THEN you will no longer be eligible for the upgrade and would have to pay if you want Windows 10.