Military rounds in general are cheaper than other ammo.
The 9mm is a military round and it is the cheapest.
The .40 is a police round and used quite a bit and it is in the middle.
The 10mm is almost semi-exotic--I rarely see it being shot; it is the most expensive.
Google "Winchester White Box prices" and you will get a better picture (practice ammo--"ball"). Or just walk into a Wal-Mart and ask their prices. (Not all Wal-marts carry guns and ammo.)