Mass-production makes contact lenses cheap and getting them online makes it easy. Since most soft contact lenses are made in a few standard sizes, they can be made in large quantities at low cost. But the manufacturing equipment is by necessity extremely precise and guarantees consistent quality when you get contact lenses cheap.

Early plastic contact lenses were hard and had to be custom fitted, which made them expensive. And even today, some lenses are still custom made and therefore cost more. But soft contacts can adapt to minor eye surface variations and so don't require the same precise fit as a harder lens. This makes them easy to mass-produce and it's one reason why you can get soft contact lenses cheap.

Another reason that it's easy to get contact lenses cheap is due to the economics for companies who operate online. Without the same expenses of your local store, online stores can sell for less. And since they have warehouses to stock larger amounts of lenses than a local store, they can also buy larger quantities of contacts from the lens maker. Which means they get volume discounts that they can pass on to you.

The reason that a contact lens requires a prescription is because it's considered a medical device. And because of this, contact lenses are made to be as nearly identical in their quality as possible. So if you get contact lenses cheap, you can be sure they're the same quality as lenses you'd get anywhere else.

Besides saving money when you get contact lenses cheap, there's another benefit. Since the lenses don't cost much, it's easy to try different brands or styles to find what works best for you. You may want different types of lenses for various situations and by getting them cheaply, it's practical to experiment.