A great many large agencies offer writers a per-word rate for their content, and some writers insist on such a rate themselves. On the surface, it seems fair enough. Putting fingers to the keyboard is work, so each completed word can be seen as a unit of work performed. It certainly takes a lot of the mystery out of how the writer came to his final payday. The downside for the client, however, is that more writing isn't necessarily better writing -- in fact, as I mentioned in a previous article, wordiness is more likely to make a piece of content less effective. The per-word rate actually encourages padding the text (at least up to a pre-agreed maximum word count), so you might end up with an impressive-looking wall of verbiage that's actually pretty weak stuff.
Billing by the hour is an extremely common approach taken by all kinds of independent contractors. Like the per-word rate, the hourly rate fixes what you're paying to a concrete, measurable variable -- the amount of time spent writing, editing and/or researching. Assuming your writer is using a verifiable program for logging those hours, you can rest assured that you're paying for X amount of labor. But there's a wild card in this deck as well, namely speed. Some writers agonized methodically over each and every word, while others blaze through the draft like lighting. To complicate things further, both writers may come up with equally fine results. So does the faster writer deserve to be penalized for his efficiency? Is the slower writer really working harder, or is he just being lazy (and racking up a bigger bill for you)?
Per-Project RateThis is the method I generally use. That doesn't make it "better" or "worse," but I've found that it serves both my clients and me fairly well. In this approach, the writer either offers a fixed menu of rates for various kinds of projects, or he eyeballs the project's scope, thinks about how much time and effort he's likely to put into it, and then quotes a flat estimate. Additional charges may apply if the project grows way beyond its original parameters. Clients like this method because they always know exactly what they're going to pay before the project even starts. I like it because it makes it easier for me to figure out whether I'm meeting my own monthly budget and financial goals. There will be some times when the work turns out to be harder than expected (meaning you get extra work out of your writer for the same money), and other times when it turns out to be easier (meaning that your writer enjoys a better payday for the time spent). In the long run, I find that it evens out.
So what kind of billing system should you look for in a copywriter? Ultimately, that's entirely up to you. But I would urge you to look at the writer's experience, integrity and demonstrated skill first and foremost. If you're getting good marketing content, you're getting a bargain!