Is Sotha Sil Expanded free?

Yes. Sotha Sil Expanded is released under Free fan mod (see the ModDB page for terms) and costs nothing. Neither it nor The Elder Scrolls III: Morrowind has any paid content.

What do I need to run Sotha Sil Expanded?

The Elder Scrolls III: Morrowind, which is itself free. Sotha Sil Expanded is an add-on and will not run standalone.

How do I install Sotha Sil Expanded?

The PlayBound Launcher installs it in one click from The Elder Scrolls III: Morrowind's mods list. You can also install it by hand — full steps are on this page.