Most people are going to tell you probably not to do it. If you have the money and really want to, then it’s really up to you to jump on in!
Check out YouTube on designing yoyos. There is one guy that has a 3 part series that is really good.
Pricing is all over the board.
Edit: the name is Zach The Sloth on YouTube. I would recommend just typing in how to design a yoyo.