Ok, I've got to ask. $15K for one text? Percentage? It's THREE orders of magnitude more other than other general text writing. How can this be justified? (Kind of looks like daylight robbery to me)
Keep in mind that writing the actual text is a very small portion of the work. 90% of a good copywriter's time is spent researching your competitors, customers, and target market...understanding your product in great detail to fully highlight its benefits...finding which psychological hot buttons exist in the specific segment of the market you want to target, and how to push them in just the right way to create a red-hot burning desire in your prospect's minds to buy your product NOW.
To write good copy, you need an absolute understanding or a market, basically all of the steps of a comprehensive market research project. This takes time and money.
You can definitely find someone to write you an article for a few bucks. It won't convert, and it won't represent your product/startup in the best way.
That's why I recommend startup founders learn the basics and write good copy themselves, You already know your product and customers best- you can skip the through research a copywriter would have to engage in.
Knowing the theory and being able to produce great work are two different things. The price is what it is because, despite it being learn-able, the experience of writing thousands of pages of sales copy, refining them and getting the craft so down to a fine art just comes from experience and practise.
In some cases, a good sales letter can be used for years and years. Take a look at this classic two-page direct mail piece that was used -- with small variations -- during 28 years and made the Wall Street Journal ridiculous amounts of money: http://www.copywriting1.com/2007/10/wall-street-journal-lett...