Okay i have created a website to sell E-Cig Stuff.
eciggistuff.co.nf

I am updating my site so people can buy things online.
When the NoNicotine Option is selected i need the Value of Amount to be 3.50.

When the Nicotine Option is selected i need the Value of Amount to be 4.00.

How do i make it so my website does this.
Thanks very much for help in advance

Here is a picture of my problem