You want to justify something that was built hundreds of years ago by the tourist attraction it provides today ?
Consider this that in the mid 17th century that the Taj Mahal was built; the British had already built the Universities of Cambridge and Oxford between 4 and 6 centuries before that and most of the colleges of the University of London in the 2 centuries after that - Education and Learning takes a Nation forward not extravagant edifices dedicated to dead lovers !
Besides a Leader ought to spend every ounce of the national treasury in service of the Nation not spending obscene amounts of money on himself and deceased members of his family !