When I work out the cost of advertising I compare it to a cost per click on adwords. So if I spend $500 a month and get 500 clicks then my cost per click is $1. Now I can go on spending $1 per click or I can pay for some SEO work to mitigate some or all of adwords cost.
SEO - is it worth the cost?
Its always when you are concern about the long term growth of your company but when it comes to outsourcing I would never do it unless they themselve rank well for keywords like SEO / Search Engine Optimization