I should have clarified - I was referring to net margin, not gross, as net margin is what you'd discuss in this context. While gross margin is useful in some industries, it isn't useful for many companies discussed on HN, and you can't compare gross margins across industries. When people say "Walmart has a thin profit margin", they're not referring to Walmart's ~25% gross margin, they're referring to their ~3% net margin. (Apple's net margin is ~18%, gross margin ~38%)
If you're looking deeper into a business's financials, gross margin can be a useful metric, but for the layman gross margins are usually used for misleading purposes. For example, gross margins can be easily Tweeted to make the point that "this evil business is making money hand over fist by selling xyz", or to indicate that a business is doing better than it actually is (Uber and WeWork, for example, have positive gross margins.)
If you want to know the number representing how much Walmart paid for that chair vs how much you paid, you want to know the "markup". Markup is the difference between the cost of something and the price it is sold for. Confusingly enough, this is represented in different ways in different industries: some represent it as a fixed amount, some as a percentage of the total cost, and some as a percentage of the selling price. If you buy widgets for $4 and sell them for $8, your markup is $4: either (8-4)/8 = 50% of selling price, or (8-4)/4 = 100% of cost.
Markup as a percentage of cost can range from several hundreds of percent (luxury goods) to ~15% (grocery stores.) Restaurants usually mark up food around 60% and drinks around 75%-400% (markup 300% of cost = $6 beer that cost $1.50.) "Keystone" pricing is when the markup is equal to the cost (meaning the ring on sale for $10k cost them $5k) - and is common in many industries including the jewelry industry, although fancier jewelers charge far more and budget sources charge less. Car dealerships, funny enough, have a lower markup than people might expect, between 2 and 5% of cost for economy base-model cars and 15% for high end cars. You can add in a few more % when you start to additional features to the car. (Dealerships these days often make most of their money through financing, service, accessories, warranties, and selling used cars.)
It's very interesting to see historically high markups evaporating with direct-to-consumer sites like Aliexpress. But I've rambled enough in this comment already :)