The total cost of maths books is 2*3 = <<2*3=6>>6 dollars
The total cost of science books is 6*3 = <<6*3=18>>18 dollars
The total cost for maths and science books is 6+18 = <<6+18=24>>24 dollars
The cost for art books is 30-24 = <<30-24=6>>6 dollars.
Since he bought 3 art books, the cost for each art book will be 6/3 = 2 dollars
#### 2