No. Local shops sell sim at face value. No discounts. Unlike other airports in the world, Singapore is an exception where most of things are at par with city prices. I heard that Singapore has some regulations that says airport price has to align with city prices.
In klook sim booking, sim pickup can be done both at airport and city stores for same price. So picking up from airport is better. Also, if directly going to the store to buy sim they say that $12 sim is out of stock (not true) and only higher price ones are available.