To my understanding, you can't purchase a domain from blogger.com. It belongs to Google.
What you're probably thinking of is registering a top level domain name, something like mywebsite.com, then forwarding your blogspot address to it.
You can do that by purchasing a domain name from Go Daddy for $10.19/year, then once it's set up, you forward and mask your blogger domain inside your new domain name.
That way your
blog looks like an official website, and you still use Blogger's free webhosting. You save
money.