I find the easiest thing to do is assume we drive some distance, then work out how much gas is used.

If we drive 1300 miles the numbers all work out easily (because it's divisible by 10, 50 100 and 13)

Suburbans get 10 mpg: 130 gallons

Priuses get 50 mpg: 26 gallons.

Suburbans @ 13 mpg: 100 gallons (30 gallons improvement)

New Prius @ 100 mpg: 13 gallons (13 gallons improvement)

So clearly the best thing to do is scrap all the Suburbans and replace each with a newer style Prius.