Return Value Based on Field Input

Hi Guys,

I need to get a value returned from dropdown list based on what is selected.

So for example user selects [1] xyz this will equal 18 or [2] xyz will equal 36 this will be picked up by the script on post and return a value to a field i.e amt =<%=Amount=>.

Any help would be great as I am fairly new to this.

Thanks.