1. The price was 40,000 won. The price is 50% off. Now the price is 20,000 won.
2. The price of the shirt was 40,000 won. The shirt is 50% off. Now the shirt is 20,000 won.
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Which one is grammatical?
The second is better, but both are grammatically correct. It'd be smoother, though, if you connected the 2nd and 3rd sentences by using "so" as the conjunction.
The price of the shirt was 40,000 won. The shirt is 50% off, so now the shirt is 20,000 won.