For under USD$100 / EUR€80, build a Raspberry Pi with a USB ADS-B receiver that can run dump1090 and PiAware. View data locally or via FlightAware Users that share data with FlightAware automatically qualify for a free upgrade to an Enterprise Account.
Professional users without an existing receiver but with a suitable facility (e.g., hangar, office, home antenna mast) with clear line of sight to the sky are eligible for no-charge FlightFeeder that FlightAware provides. FlightAware's equipment can be installed virtually anywhere; all you need to provide is power (110-240V), Internet access (via Ethernet), and installation. Data collected is automatically integrated with any of your FlightAware services.