It all depends on what the governance review is covering, who is carrying it out and to what extent. Is it a charity, or just a PC . In fact each PC carries out a governance review every year at their annual meeting where all of their policies, standing orders, committees, appointees etc are examined and amended if required at no cost whatsoever. If outside consultants are sought because of legal questions or matters then the cost is down to whatever the accepted quote from a third party charges ( and PC agrees)