edit, i cant find an exact cost but this is in the rules.

http://www.nurburgring.org.uk/ringrules.html

Accidents as well as any damage to the track, the verges, the fences, the crash barriers or any other Nürburgring property must be reported to the staff of the Nürburgring GmbH. The cost of repairing any damage and of any track closure must be paid by the person responsible. The cost per hour can be viewed on request.