Depends on the bag of Skittles you buy, but I assume you’re talking about a regular person 2.17 oz bag.
On the Target website, each 2.17 oz bag costs $0.89. 4.5 pounds is equivalent to 72 oz, so you would need about 33 bags of Skittles to have 4.5 pounds.
This means that $0.89 * 33 equals $29.37.
...
I put way too much effort into this...