Susan needs to make 30/2=<<30/2=15>>15 batches of the recipe.
Therefore, she will need 15*4=<<15*4=60>>60 potatoes.
She will also need 1*15=<<1*15=15>>15 teaspoons of salt.
Thus she will have to buy 15/5 =<<15/5=3>>3 containers of salt.
Therefore, she will spend 60*$.1=$6 on potatoes and 3*$2=$6 on salt.
Thus, she will spend $6+$6=$<<6+6=12>>12 on food.
#### 12