I think you are asking the question the wrong way, because it’s only a question you can ask yourself. I like to think of it this way: If someone came to me and said “Will you mix this song for me?” along with a price. If I look at the song and the price and get excited about mixing it, then the price is good. If I look at the song and price and don’t get excited, then the price is wrong.
Depending on the day and the song, my price might be really low. Other days or songs my price might be really high. There are other factors that determine that price as well (like how much I need money, etc)
You will need to start out being less selective. You will learn pretty quickly when a new song to mix becomes something you are looking forward to, or something you are dreading. You will get more selective as time goes on as you figure out what your own tolerances are, and if you are good, you will be able to be selective and still have people want you to mix their songs.
I know it’s not the same thing, but I approach plugins the same way. People come to me with ideas or contract work all the time. If it’s an idea that is interesting but not super exciting for me, I’ll quote really high. If it’s an idea that I really like, sometimes I’ll just make it for free and give them a copy because I want it for myself too. If it’s a plugin I really don’t want to make, I quote a number that would make me excited to make it. Sometimes that number is really high. Nobody ever takes the big numbers. That’s a win-win.
But I always can tell if I quoted well when it’s a month later and I’m 90% of the way into the plugin. If I wake up excited to work on the plugin, I know I quoted it well. If I wake up and find myself wishing I was doing something else, I know I quoted too low.