Don't know where you got the the $300 figure but Apple's website lists the Macbook Air as a $129. Source

If you want to do it yourself iFixit is an excellent source and while it's moderately difficult it's not impossible... if you go that route then pick a third party battery at the cost should be about $40-50. Any more and then it's probably easier to just go to Apple....

As to whether or not you should do this versus replace a computer I went through the same question with my wife's computer. it's a 2009 Macbook Pro and since the battery replacement it's been a perfectly fine computer. so if you're okay with the computer then by all means just replace the abttery....

But the next generation of Macbooks should be pretty nice I would think....

FDW