You will pay between $20 and $30 per plug if you buy a smart plug made by a big brand. Even if you’re going with a smaller brand, you’re looking at about $15 each if you’re not getting a good sale.
Here’s another option: spend the same amount or even less on a Tech-in Smart Power Strip instead of spending up to $30 on one smart plug. It has four smart outlets individually controllable that work with an app, IFTTT, Alexa, or Google Assistant.
Why control one device if for the same price you can control four!?