Just wanted to mention that this changes about every six months as new devices come on the market and old devices get repriced.
As of this writing, March 2019, the least expensive well-engineered zigbee pocket socket is the IKEA Tradfri. Available in many different countries including the US and the UK at a very reasonable price. $9.99 in the US, £9.97 in the UK. This works well with SmartThings, is a solid zigbee repeater including for the inexpensive Xioami sensors (which don’t work well with many other brands of zigbee repeaters), and is available at most IKEA stores. This is just a simple on/off switch, just one socket and no energy reporting, but it does what it does well and specifically is a good zigbee repeater for SmartThings.
The smart bulbs and this pocket socket from the tradfri line all work well with SmartThings. (There is only partial integration with the handheld buttons and remotes.)
https://m2.ikea.com/us/en/p/tradfri-wireless-control-outlet-30356169/
All of that said, by the Fall, there might be a different brand which has a similar price but more features. This is a very rapidly evolving space, particularly as manufacturers look at the Amazon echo plus market. 