product_variant_cost_price/data/product_product_data.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data noupdate="1">

        <record forcecreate="True" id="default_cost_method" model="ir.property">
            <field name="name">Cost Method Property (Product variant)</field>
            <field name="fields_id" search="[('model', '=', 'product.product'), ('name', '=', 'cost_method')]"/>
            <field name="value">standard</field>
            <field name="type">selection</field>
        </record>

    </data>
</openerp>