i think the subject is than.是一个定语从句。 for example , the matter is more complex than was thought. the result is better than was expected.

Interesting question. I am not sure whether my answer is completely correct, but here's my reasoning. a) Let's simplify things: Companies can't afford to give away more discounts and incentives than is absolutely necessary to retain a customer. b) Let's seperate phrases: companies // can't afford // to give away more discounts and incentives than is absolutely necessary. S V DO to retain a customer. In this sentence, of course companies is the subject (S), can't afford the verb or verb phrase (V) and the rest of the sentence is direct object (DO) c) let's simplify again to give away more discounts and incentives than is absolutely necessary. It must be clear by now that this sentence on its own doesn't make sense (it is not a complete sentence), therefore, you're not anymore trying to find the subject of the main sentence, but one of a complementary sentence. You can see this because of the "more than" that is used in the DO of the main sentence. In this particular case, I think, there is an implied "it". Maybe this becomes clear if I rephrase the sentence and d) make it a main phrase: It is important (necessary) to give away no more discounts and incentives than absolutely necessary (to retain a customer). With this rephrasing it's clear that "it" is the subject of the sentence. Because we left out "can't afford", there are some changes if we make this our main sentence. - can't afford --> has to be made clear in sentence by saying "NO more discounts..." - repetition of necessary/important is required, else the new sentence wouldn't make sense. e) the "is" in this sentence could probably be left out (hence, no subject needed). Companies can't afford to give away more discounts and incentives than absolutely necessary to retain a customer. In conclusion: - There is an implied subject (it) in the complementary sentence. - the "is" in this sentence could probably be left out