Awkward to say in English. "Their wealth is small" is slightly better.
I agree, "their wealth is a lot" is just plain wrong. You could also say "their wealth is nog big" or the suggestions below.
'Their wealth is not a lot' is basically not good english. Duolingo should not be translating literally but translating into good english.
"Their wealth is not abundance"?
*abundant
Thank you!
Their wealth is not great.
My exact translation too. It's gotta be ok.