Answer :

Just take the number away (lowest unit price) from the highest unit price.

Simple.
Here is an example because idk ur numbers; say there is 1 box of popsicles & it came with 2 that was $1.24 and another with 4 possicles that was $4.84. U find the unit price by disvising so 1.24 divided by 2= .62. The other would be 4.84 duvided by 4=1.21 so in the first box each pop wohld cost 62 cenga wich would be the greater buy. Now u just simply subtract .62 from 1.21 and get .59 or 59 cents as ur difference