It won't qualify for any free food allowance so you can work out the Syns using the formula 20 calories = 1 Syn, as hells said. For example, 100 calories = 5 Syns.
<img src="http://www.minimins.com/attachment.php?attachmentid=71605"/>
based on that nutritional info it would be about 2 syns (taken from the vita coco website)