Buy from ebay, they give you discount on new boxes. Usually seller himself posts promo code on the product page with which you can redeem even more discount.
Just look at seller ratings and see if he is legit before buying.
Your service provider may have current offers on DTH, so check regularly. I have an Airtel DTH connection, and they have the best DTH offers. The monthly pack starts from Rs 292 only and you can select the type of set top box and plan you want. You can also recharge easily and manage the account from your phone.