I'm getting the error:

Error: package.xml(purchase__c): An object 'purchase__c' of type CustomObject was named in package.xml, but was not found in zipped directory.  In the deployTest directory, I have the package.xml file below and an objects folder.  Inside the objects folder, I have a file Purchase__c.object which was retrieved from the org I'm copying metadata from.  What am I missing?  Also, do I have to name all the custom fields?  I thought I could just deploy the custom object but when I tried that it gave me errors saying that the package.xml file didn't contain all the custom fields.  Thanks.

package.xml:

<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata">
    <types>
        <members>purchase__c</members>
        <name>CustomObject</name>
    </types>
  <types>
    <members>Purchase__c.Admin_Fee__c</members>
    <members>Purchase__c.Affiliated__c</members>
    <members>Purchase__c.Amount__c</members>
    <members>Purchase__c.Comments__c</members>
    <members>Purchase__c.Fee__c</members>
    <members>Purchase__c.Fiscal_Year__c</members>
    <members>Purchase__c.Member__c</members>
    <members>Purchase__c.Other_Fee__c</members>
    <members>Purchase__c.PurchaseID__c</members>
    <members>Purchase__c.Report_Date__c</members>
    <members>Purchase__c.Report_Total__c</members>
    <members>Purchase__c.Type__c</members>
    <members>Purchase__c.VendorType__c</members>
    <members>Purchase__c.Vendor__c</members>
    <members>Purchase__c.Write_Off__c</members>
    <name>CustomField</name>
  </types>
    <version>22.0</version>
</Package>

 build.xml:

<project name="Sample usage of Salesforce Ant tasks" default="purchase" basedir="." xmlns:sf="antlib:com.salesforce">

    <property file="build.properties"/>
    <property environment="env"/>

  <target name="purchase">
    <!-- Upload the contents of the "mypkg" package -->
    <sf:deploy username="${sf.username}" password="${sf.password}" serverurl="${sf.serverurl}" deployRoot="deployTest"/>
  </target>
</project>