The price should be determined by what your customers are willing to pay.
If your customers are the people on this forum, then asking us what is a good price is a good idea. However, it’s unlikely most of the people here are your customer base. But maybe there are some.
Whether you think $50 is way too low doesn’t matter much. It’s whether your customers think it’s too low (or too high) that matters.
To figure out your price, ask and answer a few questions.
1. Who are your customers? You say people who want to start their own line but you should get more specific about this. Is it chemists who want to start their own line? Is it social media influencers that want to start their own line? Is it people who already have a brand and want to expand? You’ve got to narrow it down.
2. Where are your customers? There might be some customers here but not enough for you to make a lot of money at this. If you’re not getting feedback from the Facebook groups you’ve asked, you probably don’t have many customers there either. Do you have an email list? Do you know other people who have an email list that would be willing to rent it to you?
3. What are your competitor’s asking? Are they even selling any books?
You can find formulation books on Amazon that go for $6 - https://amzn.to/2XdOZRb
or some that go for $170 - https://amzn.to/2IHZtix
4. Will anyone buy this at all? Before you even write the book you need to to know this answer. You should also ask your potential customers what topics they specifically want covered. Just because you think gel glitter is cool, that doesn’t necessarily mean your customers will want to learn to make them.
Another thing, your background will also play into how much you can charge. A book by a “self taught” formulator wouldn’t be able to command as high a price as a book by a “cosmetic chemist with 20 years experience in industry.” Of course, this still depends on your customer. Maybe they would see this as a plus.
The bottom line is keep the customer in mind, talk to them, and they’ll tell you how much to charge. If you are not getting answers about pricing, that means you have not found where your customers hang out. Keep looking.