In terms of capability, the Rafales are a massive step up compared to the MKI. We need the Super MKI to keep up, and are expensive. And the Rafales are proven too.
In terms of service life, the Rafales are already at 7000-8000 hours and can be doubled again. MKI's at 6000 hours and you can expect another 2000 hours after service life extension. So I would recommending calculating the LCC based on a per hour flight basis.
The maintenance requirements for MKI are ridiculous. Apart from the extensive downtime due to regular maintenance, the MKI requires 3 expensive overhauls which costs $10-15M each time and takes many months. And that's without counting the 30 hours it stays on the ground for every hour of flight. The Rafale doesn't need overhaul and stays on the ground only for 9 hours for every hour of flight.
There are also a whole host of other advantages like quicker turnaround and greater sorties/day. In the same conditions, the Rafale will fly more than the MKI can. And in case something goes wrong with either aircraft, I can put the Rafale back in the air much faster than the MKI. For example, Rafale's engine can be replaced in 30 minutes, it takes 8 hours to do the same on the MKI.
The MKI is not cheaper than Rafale. The CPFH of the MKI is $12000 according to HAL. Rafale's CPFH is much lower at $10000 when being sourced from France, it will become much lesser once DRAL starts supplies from Indian factories.
You can't compare Russian upgrades with French. We paid almost $1B for the Mig-29s and that's only for 10 years of extra flying time, 1000 hours extra. We paid extra for the engines in a separate contract. We paid $2.5B for the M-2000s and are getting 5500 hours of extra flight time, so we can fly it for another 30-40 years. Mig-29s will have to be retired by 2030, we can use M-2000s until 2050-60 at least if necessary. So they are not the same.