Those that are service based usually earns one of the highest margins. The cost is usually just your petrol any maybe the place you rent and if you got workers then you just pay those. I guess the margin can go upwards of 50 %. The only thing about this is that, even though margins are high, there is a need to convince people of your service and why people should buy or pay you.
My friend earns an advertisement company doing banners, bantings, calendar publishing and many more. His business has some of the highest margin. His cost for a banner would maybe be around 20 for example and he sells it for 150. I have another friend who is a batik artist drawing all those orchids, seahorse and many more and sells each art work for about usd 150 on average. His cost is just the colours.
Online selling also contributes to high margins as you do not need any brick or mortar to run your business and you can do drop shipping which needs to be carefully studied first. Once your customer orders and pays you then you order the product. You do not need to keep any stock and you earn just doing that.
Investing in stocks and even cryptos are good business too. The only thing you need is knowledge and experience and you just pay the cost of each transaction and that's all. You are in business. Imagine you've got 10k and you just invest it and it gives you a a steady income yearly in dividends or from capital gains. That's all you need.
There are many more but these are just some. I guess there are many unlimited ways to do business nowadays with the internet boom and high speed internet.
The first product that comes to mind would be soda syrup. The cost of the fountain drink that people get at a restaurant for $2.00 only costs the restaurant around $0.20, and that includes the cup, lid, straw. The rest of it is all profit. True, there are other factors to consider such as the time it takes for someone to change the syrup, the cost of the machine, and the electric bill to keep it going... but in terms of direct costs, they're very low.
A product that has even higher margins that soda syrup is printer ink. The markups on the tiny cartridges are crazy. The printers are often sold at cost, or with a minimal markup to get more of their products out there. Some companies even put chips in their printers so you must use their ink cartridges or they won't work. This guarantees them a revenue stream, as long as you're willing to pay the exorbitant prices.
Note: According to Consumer Reports, ink can cost as much as $12,000/gallon, and many printers don't put more than 1/2 of the ink they use on the page. https://www.consumerreports.org/printers/the-high-cost-of-wasted-printer-ink/
It all depends upon the area where you live, the resources you have in your cuntry. The products which are abudantly available at your place have low profit margins.It varies from place to place. In our part of the world medicine and bakery items the margin of profits are high. The margin of crypto business and stock exchange is high but it is not adviseable to start these business without prior knowledge as the businesses are the volatile businesses.