Building a DbCommand to compare two columns on Variants
Completely untested, but this should return all variants whose price column's value is less than your custom field's originalPrice value.

$results = craft()-&gt;db-&gt;createCommand()
    -&gt;select('v.*')
    -&gt;from('commerce_variants v')
    -&gt;join('content c', 'v.id = c.elementId')
    -&gt;where('v.price &lt; c.field_originalPrice")
    -&gt;queryAll();