mascor on "[Plugin: WordPress SEO by Yoast] Displaying Custom Filed in META TITLE with Woocommerce"

Hi,
i'm using Woocommerce and i need to show in Meta Title the Product Sale Price.

I tried with this:
http://wordpress.org/support/topic/plugin-wordpress-seo-by-yoast-wordpress-seo-custom-fields-in-title?replies=10
but if i use this template:
%%cf_<field name>%%
in this way:
%%cf__sale_price%% (the meta_key in wp_postmeta is "_sale_price")
Meta Title doesn't show the price.
Why?

Thanks

http://wordpress.org/extend/plugins/wordpress-seo/