For us (retro games), it's about 50% overall. Of course, this varies greatly from product to product.
Interestingly, most of our higher value products tend to also be higher margin. We've increased revenues significantly by focussing more energy on getting promotions right for said high value, high margin listings.
>Know it depends on what you sell but always interested in launching one.
Then do it. Buy something undervalued from Facebook Marketplace and flip it, or import a small batch of a product with low local competition from Alibaba.
You can optimise from there, but starting small is absolutely a great way to learn the ropes. Worst case scenario, you've lost a couple of hundred bucks and learned a valuable life lesson.