hornady black is probably the cheapeast right now, but the say hello price is somewhere around $1.50/rd for most 450b ammo.
if you watch the classified's you can often buy some from folks who no longer need it for around $1.25 or so a round.
i think many of us load our own for that very reason
i konw the only reason i'm buying factory ammo is for the brass. New brass is around 90 cents each, and even once fired can go 70 cents or so a pop. by the time you figure in the cost of a 250 FTX you basically get the powder and primer free on the first round buying loaded ammo vs virgin brass.