$10/switch would be very cheap. Most of those will be no name Chinese imports without any safety certifications. And when you’re going to wire something into the mains, you really do want safety certifications for fire protection. 
The only option I can think of would be some of the sonoff Wi-Fi devices which go in the wall behind the switch. If you buy those directly from Sonoff they will have “C E” safety certifications. The CE Mark is a self certification, so it’s only as good as your trust in the individual company, but Sonoff seems to be pretty good.
In order to get them to work with smartthings, you will need to flash the firmware. More on that in the following thread and you can talk to people who are using these there:
Remember, though, that if you choose Wi-Fi over Zigbee or Z wave, You will run into a max devices limit much sooner. Many home Wi-Fi routers can only support 30 simultaneous devices, and the ones that can handle more are much more expensive. With Zwave you can have a couple of hundred devices, and with zigbee up into the thousands. So if you’re doing all the switches in the house, you may have to take a hard look at the Wi-Fi router you are using.
2019: Is WiFi the new IOT Standard? (CEpro Article)
Also, check specifications very carefully before choosing a sonoff device for your outlets. In the US, a regular in wall receptacle should be able to handle up to 15 A, but many devices intended just for light switches only go up to 10.
So I’m not saying it’s impossible to do safely, but you are running on a very small budget compared to what most people spend, and you are going to be pretty limited in the possible device choices.