It's "worth it" when the something gained is of greater value than the something that's been given up. In order for us to be worth dying for, it must mean that 1) we have worth apart from God and 2) there's something that God doesn't have.
The fact is that there is nothing God doesn't have. Everything that exists, from wealth to beauty to your soul, was created by God out of nothing. God can't gain anything. In Him is everything. If there was anything that God ever lacked, he could just speak it into existence; he doesn't need to trade his life for it.
Also, compared to God we aren't worth anything. The Bible makes it clear that when Christ died, it was in spite of our worthlessness, not because of our worth. Psalm 8:4 says "What is man that you are mindful of him?" Romans 5 talks about how some people would give their lives for a a good man, but only God would give his life for his enemies.
The thing that makes the gospel so incredible is that he died for us when we weren't worth dying for. That's beautiful. I have eternal life, not because I deserve it, but simply because Christ loves me enough to give it to me.
The only reason I have any worth at all is because of Christ.