You would stand to build more visitors to your site which in turn would turn into more sales. As far as the cost it depends on how many keywords you want to get first page or top 3. Most services will charge in the hundreds of dollars to rank in first page and higher for top 3. You will find some freelance stuff that is a bit lower than that.
I agree here. You should start step by step. And also search for quality SEO providers as there are a lot of fraud going on in SEO field. Best Of Luck
That's what I don't understand - how can SEO be done on PC site when we are selling over 6000 products with completely unrelated keywords
Depends on what you want to rank for. You can rank for some costly product or may use a general term, like "online computer shop" or "Buy electronics online" something like that. Would definitely increase your sales. BTW currently how do u get visitors to your website?
The question isn't really whether you want to optimise, or whether you have to. The answer to both of those is 'yes' without question. The only question that exists for any online business now is whether to outsource or do it in-house - which will depend upon whether your time or your money is the most readily available!