The cost of using Alexa on an Alexa built-in smartphone will vary depending on the specific device you are using. Generally speaking, most of the major smartphone manufacturers offer Alexa built-in devices, so it's best to research the specific device you are interested in to determine the exact cost. Additionally, Amazon offers some Alexa-enabled devices at discounted prices, so it's a good idea to check their website for any applicable promotions or deals. To get started, you can also download the Alexa app from the App Store or Google Play store and set up your device for free.