I need a new battery desperately

Hi, I have a Macbook Air from 2011 and I use it everyday for multipe hours, so obviously the batteries been depleting a little more and more over time but now I can charge it over night for a solid 8 hours, turn it on and the battery will drain in about and hour or so, a lot of the time it'll say it still has 20/30% left then it'll shut down into standby mode suddenly, so I plug it in to see it now says I have 0% and I've also found it doing this now in sleep mode like this morning I had 61% battery when I put it in sleep mode, came back 30 minutes later and the battery was completly gone.

It's been like this since last November getting worse and worse and I've read that the battery is considered a consumable so you have to pay for it to be replaced (and my Apple Care has ran out too) so I was just wondering if anyone can tell me how much a new one costs, how long it'll take to fit (will I have to leave it there for a few days), the battery won't start leaking will it, etc.?

Thanks for any help 🙂