It's actually significantly less than that generally...! I need to create a category in my commissions post for a more proper breakdown, now that I've done quite a handful....

It depends on how much back-and-forth, discussion, trials at the design you want to do with me.
I generally charge initial concepts at the price of a sketch page, $70 (loose variations on your suggestions, silhouettes, visualizing to find harmony with your mental image). Then... to finalize or synthesize based on feedback, another $70 for the final ref sheet, or perhaps more depending on how comprehensive you'd like it to be.

The thing is that we can do back-and-forth trials to get to your idea as many times as necessary, at the cost of another page each time. It depends on how particular, or how explorative, you want to be. (and you own all permutations of the design to use at your discretion).

Complexity is also a factor; if you wanted say, multiple outfits, forms, you could generally think of it as about $70/per 'page' (4 or so poses, close-ups, etc).

for a pair of characters going through this process at a bare minimum though, that'd be about $280 ($140x2 or $70x4!). It might be bumped up to $300 just to cover some poses of them interacting....!