You will tend to have different conversion rates for different keywords. You need to test as many different, highly relevant, keywords as possible. Learn the conversion rate for each and then focus on all the keywords that can result in a positive ROI.
The thing you should really focus on primarily is your landing pages. How well targeted your landing page is toward the keyword, how well the page layout is optimized, and how well written your sales copy is will determine your conversion rate. You should be split testing multiple versions of your landing page until you find a winner. You may need multiple landing pages, each optimized for a set of keywords.
The bottom line is that you need to be testing variants of every key element of your landing pages, ad text, and keyword selections. You will find that very small changes will make a huge difference at times, and the only way to learn which changes work best is by testing.
p.s. I took a quick look at your homepage if you are pointing your ads to the homepage of your website that is what you need to change. The homepage of your website is not well optimized for conversions.
