Could someone explain this paid upgrade policy to me?

If I book one way main cabin fll-lax , what would an upgrade to first cost me?

Around $300 ? when and how long prior to the flight could I do this?
It's subject to availability,so theres a risk of no seat in first?
Theres a Whopping $850 difference between economy and first on that route for the date I want.

Are you given a price for main cabin select aswell?

Thanks