Used glasses for 25 years. Most I paid on lenses was 10,000 baht. Last one was 4,000 baht. The cheaper ones work just as well. I wouldn't pay for expensive lenses again. All a bit of a con.
Normally 50/50. You don't need the most expensive lenses. 4000 baht ones (bargained price) are fine. Normally you have to change lenses every 2 to 3 years. Not worth paying for the most expensive.
Average lense is 4000 baht. A premium one is 6000 to 8000 baht. A cheap one is 2000 baht. The markups would be 100% on cost. Always bargin never pay 100% up front.