If SSL is too expensive for the Basic plan, consider dropping it altogether, or increasing it to EUR 9.95. You can also slash the free plan in half (1 project, 5 MB, 50 tasks) to encourage people to move on to paid; one project is more than enough to decide if the tool is for you.
I can't help thinking the github model might work here: publicly viewable mindmaps are free and you pay to restrict access to specific signed in users. Privacy is a better selling point than security, and some publicly shared mindmaps will generate backlinks.
