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"?
Their wealth is not great.
My exact translation too. It's gotta be ok.