Below is the computation for long term capital gains and income tax:
Purchase Year = 2003-04
Purchase Cost = 600000
Cost Inflation Index (CII) for purchase year = 463
Sale Year = 2011-12
Selling price = 15000000
CII for sale year = 785
Indexed Purchase price = 600000 x (785/463) = 1017279
Long term capital gain = 15000000 - 1017279 = 13982721
Income tax on capital gain = 13982721 x 20% = 2796544.2
As you sold a plot, section 54F can be applied to save income tax on capital gains.
Under this section, you will have to buy a residential property within two years of sale of plot or within one year before sale. You may also construct a house within three years of sale. To save tax fully, whole sale consideration amount has to be invested in new property. If less amount than sale consideration is invested, income tax will be applicable in similar ratio, in which the amount is less.
If you want to buy plot, no exemption can be taken unless you construct a house on it within three years.
I am assuming that old plot was jointly owned by you and your wife with equal share (50-50). In that case, both of you would have to invest 75 lacs each individually to save income tax fully.
You can buy two residential houses for 75 lacs, one in your name and one on your wife's. Or you can buy one house worth 1.5 crore in joint ownership.
Say, if you buy a 80 lacs property and your wife buys 50 lacs property, then there won' be any income tax payable by you and income tax payable by your wife will be = 20% of ((13982721/2) - ((13982721/2) x 50/75)).