The cheapest I think should be at MS Home user program at Rs 450/-. But for availing this you need be working or know someone who works for an organisation which has tie up with MS. (Most of the IT companies like TCS, Wipro etc.. have this tieup)
I know Office 365 is expensive if you buy it for the entire year..but if you use office for only 2-3 months then you can probably think about getting it for only those months.