You don't need a standalone Amazon speaker to control the Ecobee4 Smart Thermostat with Alexa.
The Ecobee4 Smart Thermostat costs $249.
It is only available in the US; at the current exchange rate, pricing converts to roughly £190/AU$330.
It has all of the same features as Ecobee's previous smart models.
But this version also has built-in Alexa voice control.
Adjust the volume and other Alexa-related settings directly on the thermostat.
This is the first smart thermostat to offer this sort of functionality.
This model requires a C-wire for installation. If you don't have one, use the Power Extender Kit (PEK) that comes with your purchase.
Always consult a professional if you have questions about the installation.