The Amazfit Pop Pro offers almost an unbelievable amount of smartwatch for about US$60. Despite its relatively low price, the Pop Pro has a GPS module, a SpO2 sensor, 5 ATM waterproofing, NFC and up to nine days of battery life.
https://www.notebookcheck.net/Huami-Amazfit-Pop-Pro-released-for-US-60-with-GPS-a-SpO2-sensor-and-up-to-nine-days-of-battery-life.507231.0.html