How much would it cost to get a replacement for one AirPod?

Refer to: AirPods Repair & Service - Apple Support

- Includes replacement cost (go to Get An Estimate at the bottom). Note the distinction between lost and damaged items. Neither the warranty nor AppleCare+ helps with the cost of Airpods that are lost or stolen. The cost for damaged Airpods may be lessened by any 'AppleCare+ For Headphones' coverage you may have.

I need to order a left AirPod to replace my other one

Read what I posted last year. Nobody on this forum works for Apple and we can only guide you to self-help resources.