Researchers explained that carrying out attacks against the most used default Tor bridges would cost threat actors $17,000 per month. Attack could be used for preventing users to access the popular anonymizing network or to de-anonymize Tor users with techniques such as traffic correlation. The researchers estimate that the costs to launch a DoS attack against the 38 default bridges could be of around $31,000 a month. An attack aimed at all scanners in the Tor Flow bandwidth measurement system would cost $2,800 per month and reduce the median client download rate by 80%.”]
Source: https://securityaffairs.co/wordpress/90196/deep-web/cost-tor-bridges-attacks.html