Building a Controller Extension

I'm trying to build a controller extension for the Opportunity Standard Controller, along with a Visualforce page that  pullsa method from that extension. However, when I try to save the Visualforce page I receive the error: "Unknown function ProductRate. Check spelling."

Here is the Controller Extension:

public class TestClass {
    private final Opportunity opp;
    public TestClass(ApexPages.StandardController stdController) {
        this.opp = (Opportunity)stdController.getRecord();
    }
    public String getProductRate() {
        return 'rate';
    }
}

And here is the Visualforce page:

<apex:page standardController="Opportunity" extensions="TestClass">
     <apex:outputText value="{!ProductRate()}"/>
</apex:page>