I don't remember how much I paid for them in France, but I think contact lenses are a little bit cheaper in Korea (like almost everything else).
I didn't search for more information on the Internet beforehand, I just went into the first contact lens shop "Lens Story" I knew in Hongdae, right at the exit 9 of the subway station. One of the girls could speak a little english so this might be useful.
I didn't remember how bad my eyesight was, so she just used the automated machine to figure out my eyesight. -2.00 on the right eye and -1.50 on the left eye, it was really fast.
One box of Biofinity Aquaform lenses cost 50,000 KRW but it's sold as a pack of 6. So for 100,000 KRW, I'm good for 6 months. From what I've found on the Internet, one box costs around 80,000 KRW in France. I also bought 2 big bottles + 1 small bottle of contact lens solution for 12,000 KRW. I got a free cat-shaped box to keep my contacts with a little mirror, nice! Also, since I bought 3 products, I got 3 stamps on their card, after 5 stamps, you get a 20% discount.
Buying contact lenses was very easy and there is no need for prescription. You can find lens shops very easily all over Seoul so, if you plan to stay for a long time, you can either bring your own lenses or buy them there! They use the same brands as in western countries.