Confused about why my sentence is unnatural.

Hi y'all,

I recently did some sentence building practice and asked a Korean friend of mine to correct my mistakes. She pointed out a sentence I wrote using (으)ㄴ/는 셈이다 as unnatural. However, a very similar sentence from my textbook she thinks is natural. When I asked her why they feel different to her, she couldn't explain. So here I am to ask the native speakers of this subreddit for help. I really don't get it. :D

My sentence practice:

가: 이 옷은 진짜 비싸네요.
나: 브랜드가 유명하고 품질도 좋아서 비싸지 않은 셈이에요.

The sentence from the textbook (Korean Grammar in Use Intermediate):

가: 여기 구두는 다른 회사 구두보다 비싸네요.
나: 품질과 서비스를 생각하면 비싸지 않은 셈이에요.

Maybe someone can explain why my version is unnatural? My friend said it's technically correct, but it feels awkward and not like something a native speaker would say.