The <p:commandButton> submits and processes by default the entire form. This will indeed validate all input fields. You can control this with the process attribute which thus defaults to @form. In your particular case, you could just use @this so that only the command button's own action is invoked.

<p:commandButton value="Add another price" process="@this" update="pricesList" action="#{productBean.addNewPrice()}" />