The Fitbit Air has entered the chat. Will Shanklin / Engadget Fitness tracker pricing can be, well, a bit misleading. For example, say the $399 Oura Ring 5 seems to be within your budget at first glance. But to make it remotely useful, you'll need to pay an extra $6 per month or $70 per year. Your total cost jumps up to around $550 after only two years. (And if you hang onto it for five years, you're looking at at least $750.)  Oura isn't even the most egregious subscription-based offender in this space. Whoop, the popular screenless fitness tracking band , requires a membership starting at $199 per year. Granted, that includes the device itself, unlike others. But you don't need to be a math whiz to realize how quickly its cost can add up over the long term, too. That's why Google's recently released Fitbit Air is so interesting. It's basically a Whoop, with a one-time cost of $100. You can use most of its features without an additional subscription. What a concept!…