Translation:Those clothes are not expensive.
Tips in remembering the kanji:
Something that is expensive and high is usually talked about often (mouth particles).
Possibly because an outfit is a set of clothes that are meant to go together as a whole, while ふく is just clothes in general. But mostly because the creators didn't think of that solution yet, probably. If you flag it, they might add it to the database (or not).
What is the difference between the plural and the singular? Why is "This clothing isn't expensive" wrong?
Because the word "clothes" is plural, and the word "that" is singular. You want "those clothes", or if you absolutely want it to be singular "that piece of clothing is" (but that one might not be in the database yet).
Yes, because of the use of "その" specifically implies "that (clothing) over there". Without その, you would be correct
Yes, I made a comment above, but for reference it's because the use of "その" specifically implies "that (clothing) over there". I.e. it's saying "those clothes are not expensive". For it to say "these clothes...", you would need "あの" instead of "その".