How much it may cost if I decide to use some SEO expert to bring my website to top 3 or 4 for a very competitive keyword ? I am thinking about search keyword like "antivirus 2013 reviews". Or can I even do the seo by myself ? And how do you decide guys if a keyword is very competitive or not ?
Its bit expensive to be at top and cost depends on their experience and work. By using adwords and trends in Google you can able to find the keywords competition. To get short term result its must to hire a seo for your work.
First of all make sure that you are going to hire someone or you will do yourself? And also as you have said in your question that you want to hire some SEO, I would like to suggest you to hire only One expert rather than more than one. Because if you hire some SEO experts for your site, then you might spam your site due to huge SEO work on your site instead of getting good ranking position in search engine. And also it will be very expensive to hire some SEO experts, If you want to do it yourself then it is not a difficult task. You can save your hard earned money by DIY. Just learn SEO, there are large no of resources present on Internet to learn SEO.