Basically, to find the best deal for the price, you have to find the price per unit, or in this case, pound.
So just break it down,
Wegmans 30 lb bag- $5- goes to 16 cents a pound
Tops 20 lb bag for $3- goes to 15 cents a pound
and Costco 75 lb bag for $12- goes to 16 cents a pound as well
Then find the rate from the results of cents per pound
I hope this helped in any way, good luck!