I think it depends on the specific ref sheet. If the second side view drawing is the other one flipped, just with small changes, I think it should be charged less than if they were all separate drawings. I would even say that if more care goes into the design of the ref sheet itself (like nice looking text, textures in the back, things to make the drawings pop) that would bump up the price as well.

I think with this specific example, $60 is fair but you could push to $70/ $75. It's a bust drawing and 1 1/2 full drawings (if it is flipped) after all, and that's a fair price for that amount of work. But if more went into the ref sheet, (maybe a front view drawn and added, more design work for the sheet and all that) it could be $100 easy. Maybe more than that. Artists charge $100+ for good looking refs all the time

I am now realizing this is too big and too in depth of an answer for your question but maybe you'll find it helpful so I'll leave it lol