Standard DisplayFusion licenses are per-user, per-PC. For home use, you can install on up to 5 computers you personally use. For business/enterprise, volume licensing is required.

If your budget is truly zero, consider open-source multi-monitor tools:

These won't match DisplayFusion's polish, but they're safe and free.

DisplayFusion has appeared in software bundles from:

These bundles often sell DisplayFusion Pro for $5–10 as part of a larger package. Subscribe to deal newsletters to get notified.