The Official History estimates that 33,000 men were taken prisoner at Tobruk, when the fortress was surrendered. The German casualties in the campaign to capture Tobruk were about 3,360 men killed. South Africa lost about one-third of their men in North Africa was prisoners. The German practice of officers leading in combat led to high casualties (perhaps as much as 70% in the motorized infantry and armoured units).
The main reason that the fortress fell was that a decision had been made as far back as February 1942 not to allow Tobruk to be besieged again. Because of that, the defenses were in poor condition. On top of that, the 2nd South African Division was not suited to defend the place, as the commander and men lacked the necessary experience.
With the surrender of Tobruk, Rommel was promoted to Field-Marshal. He expected to be able to blitz all the way to the Suez canal. This is based on the account in Vol.III of the Official History.