Translation:This coat is not cheap.
if it didn't work it's because Ain't technically isn't a word despite people saying it, it's dumb that ain't ain't considered a word
"This coat is not inexpensive" should be accepted. Cheap carries a more negative meaning than just not expensive.
"Inexpensive" means not costly. "Cheap" can mean inexpensive, or of low quality.
I'm confused, the hint for 'yasuku' said it meant "small in quantity". How is that equivalent to "cheap"? To me it sounds more like it should mean "there aren't many of these coats", but I guess not?
This is Duoingo being finicky; "small in quantity" is the hint for すくない, which you get if you hover over す. If instead you hover over や, you get the hint for やすく, which is "cheap".