"the cost of this ad is very low" was rejected but I think it should also be accepted
I don't really speak Mandarin but I think you have changed the sense of the sentence by putting it into good idiomatic English when what was required was duolingo-approved Chinglish. The cost wasn't very low but simply low
This advertisement is low cost. Reported may 7 2018
Probably because the subject of this sentence is 费用. The 广告 is adjectival, as indicated by the 的.
It should accept for as well as of. So frustrating..