First of all, it's a fact that if you have an app open, close it (swipe up to kill the app) and then restart it, it WILL use more battery than if you simply exit out and go right back in without killing the app.
With that said, how much is actually wasted? To be honest, I close stuff all the time and I've never noticed a negative impact. I'm not saying is doesn't take extra juice, I'm just not sure it's noticeable. Perhaps if you added them all up from an entire day you might notice a difference. Or maybe not.
Personally, I think this falls into the same category as things like turning off bluetooth, wi-fi, etc. to save every single electron possible. I'd rather not micro-manage my phone and prefer to just use it however I choose. Power is easy (for me) to come by, so I'm not concerned if re-opening email throughout the day uses an extra 2% of my battery. It's not like I can't get through a day, so I honestly don't care about best practices in this scenario. I'd rather charge more often than worry about what might be running at any particular time and eating my battery.
If anyone has any hard evidence that killing and re-opening actually does eat up a significant portion of your battery, I'd love to read more. But in all my years, I've never seen such evidence. Just the same old thing from Apple repeated over and over and over and over.