Mike,
Here are my thoughts...
So does that mean you would charge roughly the same to write a letter selling a $40,000 product as you would to sell a $19.97 ebook?
From the point of view of your client, wouldn't one letter deliver more value than the other?
Shouldn't you be compensated accordingly? According to the value you deliver, not the service you provide?
How can you price a service before and disconnected from the VALUE you are about to provide to your client?
Plus... doesn't some of your value come from creating a UNIQUE solution to your client's marketing "problem?"
You should be compensated for that as well. And you can't really set a price for that until you know what THAT is
What about coming up with a better way to weed out the tire kickers WITHOUT putting a self-imposed cap on your income?
As a professional, the value you provide is worth more than that.