As with other premium crops in the game, these premium sweet potatoes cost 5 City Cash per square that you'd want to plant. This will allow you to donate as little or as much as you'd like, simply by planting more squares. The sweet potatoes are an instant growth crop, giving you 250 Goods per square planted, and they will never wither, no matter how long you might leave your game unattended.
If you don't have any Farm Cash to donate in FarmVille to unlock the Daikon Radishes, but you do have extra City Cash lying around, this is a great alternative to still help those in need. You can also look into purchasing a Kobe Cow in FrontierVille, if you'd rather donate with Horseshoes instead.
Will you purchase Sweet Potatoes in CityVille? Let us know in the comments.