That is personal anual income and not family income I suppose. Remember that India has around 30% of people far too young to use cellphones. A family might have one phone and will happily spend $28 per year for the $30GB data per quarter. Also it is mostly prepaid with hundreds of plans to chose from so people chose based on their needs.
Smartphones are extremely important part of India's way of life as almost everything else is regulated to death. It is a ray of light for most poor people and you will find people who might not have a toilet but will own an android smartphone.
By 30 GB do you mean the 1 GB per day (for a month) plan which some carriers provide nowadays? I've seen it cost in the range of Rs. 350 to 500 from some carriers, which sort of matches your numbers about GB and price.
My uncapped 100Mbps link costs me 0.7% of my monthly salary.
The plan you mention, is about 11% of the average salary in India.
Even though it's cheaper in absolute terms, it's still more expensive in relative terms.
I actually saw this mentioned in a recent UX talk by google: the salary differences end up making internet a lot more expensive for the locals in India, Indonesia, etc, to the point where some apps actually show how much they'd use (eg: Youtube shows the size of a video before downloading).
People still micro-manage they available data because of how expensive it feels to them.
The 20-30 Mbps speeds are restricted to some areas. Urban areas such as Bangalore, Mumbai and Hyderabad enjoy great speeds in broadband as well but for the majority of indians, reasonably fast and cheap internet connectivity means Jio. Also, the 20-30 Mbps speeds are not the norm outside certain areas but they have been able to cover a reasonably large part of the entire country.
One carrier (Jio) has a 4G only network throughout the entire country, no 3G or 2G - benefit of coming in to the market late.
Copying this, almost all carriers are in various stages of decommissioning their 3G services and migrating to 4G countrywide. So, even in remote corners of the country, 4G is more widespread now but the speeds are not always 20-30Mbps across the country.
The prices posted by OP seem to be referring to the carrier Reliance Jio. Their network speeds were great initially(20-30mbps when other carriers we're hardly at 5-10mbps). The speeds have deteriorated significantly now. The network seems to be reeling under the load of it's huge user base.
It depends on the area, I have used it in multiple cities and except one location (problem with all networks there) all else was good. There is an option in the jio app to register complaint about slow network, I have had some luck with that.