Changing One Form Field On A Page With Multiple Forms

I have a page with 2 product forms to add to shopping cart each form has the same fields.
when someone changes one of the dropdown boxes depending on the size it is to change the price to a certain value. This works ok whan you only have one form but now that there is two forms. When you change the second form dropdown box it changes the price on the first product.

OnChange='if (this.value<=16)ChangeHiddenPrice(90);'
I suspect it has to do with how i reference the form field

function ChangeHiddenPrice(newprice) {
document.getElementById('PRICE').value=newprice;
}