$ twurl "/1.1/statuses/show.json?id=687019753651748864" | grep fav
"favourites_count": 325,
"favourites_count": 28,
"favorite_count": 3818,
"favorited": false,
"favorite_count": 0,
"favorited": false,
The third of these values (3818) is the favourite count on the original Tweet that was being retweeted.