Beyond their regular and fairly expansive shake menu, Sonic is currently offering Holiday Mint Shakes in chocolate or vanilla flavor.
Sonic's shake prices start at about $2.29 normally.
I've always wondered why it's always 'half-price shakes" and not something warm like 'half-price hot chocolate' or 'half-price coffee.' It's the dead of winter and Sonic isn't known for having indoor seating (granted that is was around 80 degrees earlier this week in Southern California).
Here's the TV spot for the promo (I like how it's nice and sunny out):