How do the following phrases differ from each other?
Was kostet ...?
Was kosten ...?
This is a sample phrase I found in a book:
Was kosten 100 Gramm Käse?
Was kostet ein Kilo Rindfleisch?
I can’t tell the difference between the two.
This is simple concord of number. It works exactly the same as in English:
A pound [singular] of beef costs $10.
Two pounds [plural] of beef cost $20.
(You're probably surprised because German can often have the subject after the verb. That's not exactly like English.)