You can get cheap RM10 sunglasses. Frame and glass included. Examine them closely. Some of these have frames similar to prescription glasses. They may not have extras like springs, etc, but all these stuff do not justify pushing the cost from RM10 to RM300-RM500.

RM20 to RM30 cost for the shop sounds reasonable. There is some profit from the actual lenses too, but I suspect it's much higher on the frames. The economics of running an optical shop is like this. Unlike some other businesses that might get a hundred or more customer a day, you will only get a few customers daily. Instead of earning a few RM from each customer, you have to earn more. The only way is to earn a lot on the frames and lenses.

Unfortunately, this is also a weakness. This extremely high profits is a very attractive target for a profit destroying, market disrupting company. They could aim to get high volume on low margins. There are online shops that charge very low prices on frames and glasses. Mostly they're in the US and Europe. When one opens up in Malaysia, it will cause difficulties to the present shops that rely on high margins.