Why is it so confusing?
This is how it will happen.
The DTH guy will say: I am going to charge capacity fees of Rs 75 per month (assume) for first 100 SD channels (equal to 50 HD channels). For every 20 more channels, I will charge Rs 15 as capacity charge (assume).
After this, the DTH provider will also give various packs.
One pack will be purely FTA. This pack is likely to contain all FTA channels on the platform. In case of this pack, the DTH provider will only charge capacity fee, nothing else. This can contain more than 100 FTA channels. It is not clear whether the capacity fee charged on this pack should be equal to the capacity fee charged on other packs or not (most probably, it will have to be equal). If it is equal, then you're screwed, because if the pack contains 250 FTA channels, you'll end up paying anywhere between Rs 200-300 as capacity fees. If, on the other hand, there is no requirement for uniformity of capacity charge across packs, then the capacity fees may be minimal (say 99 rupees), to encourage people to come to the platform.
Similarly, there will be other packs from the DTH provider. I believe a single pack cannot contain FTA and pay channel. So, there will be packs containing only pay channels (say only Tamil pay channels), and there will be packs containing only FTA channels (say Tamil FTA channels).
You can even go for these packs as your 'base pack', and don't have to go for the 'main FTA pack' (250 channel one). You can also subscribe to 4 or 5 such packs to make up your base pack. You can also add single channels (a la carte) to this 'base pack' as well.
Now, is there any more confusion?
