Translation:The cost of this advertisement is low.
"the cost of this ad is very low" was rejected but I think it should also be accepted
Personally, I don't think so. Based on the inclusion of 这个, the word 广告 (as it's used here) probably refers to a specific advertisement, and not to advertising in general.
Probably because the subject of this sentence is 费用. The 广告 is adjectival, as indicated by the 的.