In my opinion, cam shaft does not just breaks on an odd bad day, rather it needs series of events which leads to such a mishap. If one replaces the cam shaft without isolating the events that triggered the breakage, it is bound to break again after some time. Experience says that if some work on the head is done like facing, valve replacement etc it leads to cam breaking if these jobs are not executed carefully by an expert mechanic.
So my suggestion is to first find the reason and all possible causes. What all jobs were carried out on the head portion in recent past? Oil / filter changes other than normal routine? Oil pump issues in recent past? Over heating issues?
As far as cost is concerned getting entire head changed with a good condition kabli head would be more feasible (Rs 10,000/-) in the long run rather than only cam (Rs 5 to 6,000/-) change, especially if the cause of breakage comes out to be some work on head in recent past. Otherwise, it may break again!