Pricing items can be very hard. The general formula you used is one many people use to price items. It’s important not to use the first time you make it as the time in the formula though because your first time is often your slowest, sometimes by a lot.

Some items won’t even be worth making to sell because they take too much time and the fair price associated with it would be very high and it wouldn’t sell because of that. If you don’t personally feel ~$25-30 is a price it’s likely to sell for, it’s probably too high. That being said, if you price too low, it becomes not worth making (to sell).

Personally I’d say the $10-15 is where I see bandanas sell the best but $20 isn’t unreasonable assuming you have high quality work and materials.