Prices in shekel
Cheapest donuts: Ariel bakery plain with added strawberry flavor somewhere in there. Don't know what it costs, but every Yeshiva orders it by the hundreds so it has to be the cheapest.
Best balance of price and taste: Sova bakery. Strawberry: 3.20 Parave caramel (whatever that is) or vanilla with chocolate cover and sprinkles (very popular by the kids): 5.50. This was also the most expensive donut I actually ate. (Rent is due at the end of this week and I was still short over Chanukah.)
Most expensive donut I saw for sale was New York Bake Shop (by the Mir), 22 shekel. Didn't pay attention to what the flavor was because I had no inclination to buy it.