DubScript is a free download and ALL features (import/export, etc.) are, as of this writing, free to use without restriction to number or scripts, number of scenes, etc. There is no advertising when in "view" mode (that is-- when reading screenplays, you aren't interrupted by ads), but there is an occasional ad in the "write" mode and on a few screens.
For a very nominal fee, you can turn these ads off (available in-app). Subscribing to remove ads also removes the small DubScript watermark header in PDFs and cloud-printed documents as well.
If you feel the need to make a donation, however, how about to the Electronic Frontier Foundation? They help keep the Internet free of evilness from government and corporate interests alike. You can read more about what they do on Wikipedia. If you haven't contributed to them before, you really should. In the US, it's a tax-deductible donation.