Agree with Keri.
I don't think there is a "tool" for what you are looking for. Every site will have different conversion rates for the same keywords, based on relevance, reviews, authority, etc.
BUT
You can start to develop a good idea of keyword/conversion rates for your site over time.
You need to start thinking about the searcher's intent for a query as well. If someone searches for "Rayban Sunglasses" What % of them are ready to buy? What % of them want information on Rayban Sunglasses, such as prices, reviews, styles, etc.
I would imagine a keyword like "Rayban Wayfarers" will have a higher conversion rate than "Rayban Sunglasses."
Further, I would imagine "ray ban wayfarer 2140" would have a higher conversion rate than "rayban wayfarers"
From my experience, the longer tail, lower volume keywords usually have a higher conversion rate than the high traffic generic keywords (although when you are ranking for the generic terms you may turn out more conversions than the long tail, just due to the volume)
Hope that helps 