This is a service that IMO is sorely needed for those who don't have the budget to hire an advertising company (who'd tend to get their interns to write copy for you, and at more than $175 for a landing page). That said, I'm a little hazy on what classified as a "landing page". I could see that one size fits all pricing coming back to bite you, as not all clients will have an equal opinion on what's a reasonable amount of copy, but I'm assuming that it's a calculated risk that you're taking there.

I could see it being quite a difficult service to get off the ground. The barrier to entry may be (comparatively) low, but I couldn't imagine clients being happy with you taking credit for the copy that they buy even on your own site - do you have a strategy to handle this and get people talking about the service you offer, or are you planning on relying on banner/text ads to bring in new clients (or is it something that's not entirely sorted out yet)?

Either way, all the best, and good luck, I hope it works out for you.