He bought the house 5 years ago, for 30 million rupees. As bob_chasm wrote, army officers would get plots at discounted prices as part of their compensation package. A long-term officer like Musharraf would be able to acquire a fair amount of good land. That land would be worth a lot today, and could be sold to buy the plot for his dream mansion.
Daily Times - Leading News Resource of Pakistan
The report said that when Musharraf bought the empty plot in 2003 it was worth about Rs 30 million rupees (£ 230,000), based on land prices in the area at the time. Today the house and the garden are worth Rs 120-140 million, making it a good investment even if he has to sell it or rent it out, the paper concluded