Hi,
Normally, it already takes the price of the main variant price.
In product / show.php, the rich snippet uses this code:
<meta itemprop="price" content="<?php echo $this->itemprop_price; ?>" />
And $this->itemprop_price is defined in the product / listing_price.php view file called a few lines before on the main variant.
That code runs except in the variants are displayed as a "list". In that case the rich snippet for the price is not provided.
Could you provide the URL of a page with the situation ?