The needs analysis is free as well. The difference is the benefit of using your product/service. If the letter's focus is a measurable benefit it will have a far greater ROI then just telling potentials the product/service.

I am not suggesting any "illusion" because no business should be based on a lie, but a measurable benefit, any benefit that is a direct result of using the product/service. Performance tuning is a very measurable benefit based off of time. It's what I have done for the past 7 years on AIX/UNIX servers. Regardless of what you choose if you can't measure the benefit of using it to your customers aside from "it's better" then it won't achieve the success it could.

The psychology of buying is that an emotion is what motivates someone to purchase and then they justify the purchase logically so start with the benefit of a product/service as a result of using it.