The audit services branch is mainly remains outside of Russian researchers in the transaction sector. However, the content of processes and relations, in particular in the field of contracting, is quite interesting from the standpoint of the New Institutional Economics (NIE). In this article the specifics of contractual relations in audit firms from the point of view of NIE are analyzed. In particular, the specifics of audit services as credence goods and presented the main trends of recent years and features of the Russian market of audit services are discussed. The authors have developed the methodical approach to an estimation of transaction costs of audit firms based on the classification of transaction costs by R. Kapelyushnikov.