Tata Play only offers discount on long term plans(annually or half yearly) that too sometimes. They will always try to force you to buy a higher speed plan by offering discounts on them, no offers on the same speed plan.
Tata Play is not cheap by any means, infact they charge a little premium compared to Jio and Airtel on monthly plans.
The cheapest ISP is the infamous and forum's favourite "Excitel" in terms of plan speed.
In my personal experience, I'd categorize ISPs as follows:
1. Category A ISPs with good routing, consistency and uptime(Subjective): Airtel>>ACT>>Tata Play
2. Category B ISPs good for casual streaming and browsing: Jio, Excitel and Hathway
I'd pick Category A ISPs as primary anyday if they serve my residency.
Imo, download speed is not the only indicator of a good ISP. There are a lot of other factors which are subjective and varies from area to area.