Let's assume $45 is correct for materials. Now you need labor to assemble it. You have spent time designing this power supply. It may seem trivial, but you need to consider heat, performance and reliability under different power line and loading conditions. Of course it will be compliance tested by a lab. How are you going to market the power supply so people know that it is available? Will you buy a magazine ad? Don't forget to write a manual. And people will call you with lots of questions, so plan on plenty of time for this. What cables are included?
After this and much more, you will sell it to a shop and hopefully make a profit. Then the shop will sell it again to the end customer and also make a profit. Do you still think it can sell for $90?
DIY is great. I started experimenting with electronics when I first became involved in ham radio at age 13 and never stopped. It's fun, you learn a lot, and it feels good to use things you have made yourself. But it's not for everyone. If you don't have fun, it is no bargain.
Regards,
Josh Fiden
