multiple level select field "special" version: counsel needed!

hello, I was wondering what is the better method to do this:

create "price" filed for an event which should work like this:

option1: free option2: payment due

if option 1 "free" is being chosen, the value "free" is being inserted

if option 2 "payment due" is being chosen instead, then a dropdown menu open up with a list of numbers from 1€ to 1.000€ so he can select the exact price, he selects eg 150€ and the value "payment due: 150€" is being inserted

at first I thought about using multiple level select field, but then I realized that it could nopt work well t display the list of numbers from from 1€ to 1.000€

do you think I have other possibilities for what I want to do?

thank you