Formula where we can enter an amount and then calculate a cost

Can you help me to make a formula where we can enter a 'coverage amount' and then calculate a 'premium cost'?

I was able to make a trendline here on this graph if we have a "coverage" field that says 200,000, then we need a calculation that spits out ~1000 or whatever the premium would be based on that trend line does that make sense?
I can make the equation, but I do not know how to turn it into a formula where X is the coverage amount...