Great question! It depends on what you're including with the challenge (example: workouts, nutrition, 1-1 accountability, etc.). You want to attract your ideal clients who are also financially qualified to continue on with your core program once the 21-day habit challenge is over.
As general rule of thumb, 25% higher or lower as that same amount of time would cost in your core program is a good place to start. Example: If your core program is $100/month. That works out to about $25/week. Therefore 3 weeks (21-days) would cost about $75 in your core program. 25% higher or lower for the challenge would be anywhere from $56.25 to $93.75. It's not a perfect science, and you've got a little give. So if these were my price points, I would charge anywhere from $49 to $99. (And if it were me, I would go higher instead of lower!)