Honestly that seems reasonable, I used to charge $60 way back in the day but now with newfangled Windows 8 and 10 I find it's harder to do stuff in windows now so I'd be tempted to charge $100.
It's not just the simple act of factory resetting that's long, it's having to backup all the data before such as configuration, actual data (music, pictures etc) and then restore it, reconfigure everything etc. Bookmarks, email, any other applications they had etc. Anything that auto remembers passwords they probably won't know what it is, so have to hunt that down as well.
Typically I image the machine before I do anything to it, then image it after I'm done so that next time I have to do it I just restore that image. If I missed backing up anything then at least I have an image. Though sometimes I just rsync the entire thing over instead of an image as all I really need access to is the raw files. But all that takes some time even though lot of it is just spent waiting.
I try not to deal with that anymore though I find it's just not worth my time and I'm not as well setup for it as I used to. if I lost my job I'd consider getting back into it though but even then, there's just not enough demand to make it worthwhile. All the money is in phones and tablet repair now and each one is different and requires a whole different procedure to deal with so even then it's not really lucrative. When anyone asks me about that stuff I just say I don't do it at all and to go to one of the places in town.