Need your help deciding whether it'll be worth it

I have an ios app that I want to promote. I created a website for it and as of now, it's not ranking for anything. I contacted an SEO expert and he gave me around 7 keywords that he could guarantee me the top 3 spots for within 2-3 months. However, the combined traffic of all 7 is only around 4500 monthly searches, with 1300 for the highest keyword and some keywords slightly non-relevant to my app (let's assume 4000 monthly searches). I'm trying to target U.S. customers, so only around 70% of the traffic according to google's keyword planner would be what I want; so that would amount to around 2800. Around 60% of the traffic is done from mobile devices, so that would become around 1680. Then there's the fact that my app is only for the ios, which basically cuts it down in half to bring me around 840 daily searches. And then there's the conversion rate, which I have no idea as of now. Of course, this is probably the most pessimistic guess as other traffic from other countries would probably help me generate revenue. I might be able to get an Android version out soon, so hopefully that'll bring it to at least 1680 monthly searches. Do you think it'll be worth it to perform SEO on this site? The numbers look rather abysmal.

Can anyone also tell me about what percentage of traffic usually checks out the second and third spot in the search results? I would assume for the first spot that it would be basically 100%.