I'd suggest posting on
this thread for help pricing art. I'm not going to give specific pricing suggestions here since it better belongs on that thread, but I will offer some general advice;
First, I want to note that on CS we do require that any art shops must have a CS payment option available. It is fine to allow real-currency payment, but there must be a CS payment option too. And all real currency exchanges must happen offsite. It is fine to say how much will be paid here, but any exchange of info needed to actually send the money is considered personal info and cannot be sent on CS, publicly or in PM. For example, a user can post saying they will pay $10, but after that you'd have to move to another site like discord or dA to exchange paypal info, email, etc.
With art/art shops in general, I found setting price was easiest done by looking around for other art shops with a similar level of skill/look as mine and see what their prices generally were, and trying to price near that. Personally I had initially valued my art fairly low and started upping my prices as I started getting far more requests than I could fill, and I've found thats generally a good way to figure out your pricing if you are unsure or worried about overpricing (though in my experience most people underprice their work). If you are getting tons of people wanting your art, up the price, if you don't seem to be getting any, lower it a bit. It might seem like, upping the price too much will drive people away or make it inaccessible for some people who might be interested, but its important to remember to value your own art fairly as well, I usually combat this issue by just having a wide variety of payment options available.
Getting started can be difficult since there won't be anyone following your thread at first, so don't be discouraged if you don't immediately get commissions, give it a few days. If that does happen, I just suggest bumping your thread every time it falls off the first page of the art shops board to keep it visible, which is something that really would only need to be done once a day or so, so its easy to keep up on and helps encourage interest. If you actively post elsewhere, you can also put some of your art in your sig along with the link to the shop to help point people that way.
Specifically for selling adopts/designs, I don't have a whole lot of experience there but from what I've seen, building up a customer base is as important (if not more important) as having quality designs, so it is very difficult to judge how much would be paid for a design as a design by one person could go for much less as the same design by another person, simply because of who is selling it. Just something to keep in mind, that is mainly observation my part as I do not have as much personal experience in selling designs. I think most of my above advice for art shops would still apply for designs/design shops though so I don't really have much to add in that area.