You are right that in valuation terms the stock does look attractive at around 13 times historical earnings. But you must look at the latest quarterly earnings. BSE reported a 20% fall in net profits at Rs.41 crore for the June quarter on the back of higher expenses. Total income of BSE for the June quarter was almost flat at Rs.166 crore.

The BSE has recently stated its intent to buy back around 67.7 lakh shares at a price of Rs.680, which is nearly 40% higher than the current price. Even if you factor for the proportionate acceptance ratio, it is still a good premium over the current market price. Even if you leave the buyback out, you can still look at buying the stock at current prices as the downside risk on the stock is quite low. Ideally you must keep a time frame of a couple of years.