Nagios is free and opensource software . However there is a commercial offering .

If you buy Nagios license ( Costs Qatari Riyasl 5500 ) you get one year email support from Nagios ( apart from local support for our clients in DOha Qatar ) and updates . Nagios is perpetual license you need not renew . But support and maintenance upgrade entitlements are limited and can be downloaded . Annual support renewal costs QR-1000