We can find the Unit Price of the box.
Both boxes will have the mass and price printed on them.
Suppose firat box if of 2 kilogram and costs $10.
While second box is of 5 kilogram and costs $25.
We can divide the price by mass and find the unit price.
So
Unit price = 25/5 = $5 per kilogram
Similarly
The other one also gives the same answer
10/2 = $5 per kilogram
This is how we can compare the prices using unit rates.