I would personally find that to be extremely unethical, and I wouldn’t accept anything built for that reason. If you’re going to build something, build it to fill a need and do it really well because people want it. Don’t do it just to build knockoff products to undercut people that work hard. That’s not competition.
If you think the prices are too high, contact the creator and talk about it. Maybe they’ll listen, maybe they won’t, but if you’re a potential paying customer, getting input like that is really important and valuable.