1. Low-cost-airline does not mean the airline has spent less on aircraft, or skipped on maintenance.
In fact, at least in Europe, some of the best maintenance available in the industry is by the low-cost carriers. I’m not a business analyst so I will try to explain this a bit more simplified:
The reason you get low ticket prices is because low cost carriers fly (relatively) brand new aircraft, with a very low operational cost. Meaning, per hour of flight, the airplane is cheaper to operate than the older not-so-economic ones. These new aircraft are more pricey though, so you end up paying a lot on the montly lease. What you end up with is a fleet of aircraft that cost a lot of money (montly lease) to own, but pretty cheap to operate. And, mind you, you have to pay the lease whether the plane flies or stays on ground for maintenance, so the name of the game becomes: keep the planes flying as much as you can.
Compare this to a flag carrier that owns all of its aircraft. If a plane stays on the ground, it is not making money, but it is not costing anything either.
2. Paying for good (preventive) maintenance ends up being a lot cheaper for most low cost carriers.