Just like the Latin alphabet you're reading right now, the SignWriting alphabet can be used for free. You don't have to pay anything. Just go ahead and write!
The SignWriting computer font is licensed under the SIL Open Font License (OFL). This license lets you use, modify, or redistribute the font, for free and without any acknowledgment.
However, if you develop a software application which embeds the SignWriting computer font, your application must include the font's copyright statement, the license notice, and the license text. Also, you can't sell the font to other people.
The license has other rules, but these are the main ones.SIL Open Font License