a) They could take a percentage based on your price. When it sells you get your $40, but the retail price the shop customer paid was $52. It seems that 25% to 30% are the lower end of the shop's commission, 50% is quite common.
b) You tell them what you want the retail price to be ($40). Then they tell you what percentage they take. You price it at $40, they take 50% - that means they give you $20 once the stock is sold. This is why you need to know how much you want for your pieces, and how they arrive at their retail price.
c) They sell it at the price they think they can get, maybe $100, and they tell you what margin they need to make, say 50%. That means you get $50 when they sell it for $100.
… or any combination of any the above. In any case, it is important that you have an open and honest discussion with your gallery or shop, and then make a decision after that. No need to rush these things. And you can change. Work with your retail shop for a few weeks or months or a season, see how it goes, and you may want to adjust your prices up or down.
After that, think of your customer: is it a ‘playpiece’ to wear and discard, or is it a special item to be taken out on special occasions? Think of the shop, and what kind of customer the shop has. How much money do their customers have or do they spend? Where are they coming from? Is the shop selling to students, farmers or cruise ship tourists? These are all questions that your shop owner might (or might not) want to talk about - and eventually price accordingly. That’s why you need to think about these questions as well.
The Terms You Have agreed Upon
Pay your Tax
There is more information in the free guide by Beading Daily: How to sell your Jewellery.
Hope this was of help and you enjoy the process!