I don't think there is any bargain basement prices on .454 Casull ammo. Check with GBO Sponsor Ron Reed of Reed Ammo and see if he offers it. His prices are as reasonable as anyone's on what he sells. I can't stay up on all of his offerings but he is a good source at fair prices for the items he supplies and I'm fairly sure he can provide what you seek.
Of the major brands I think Magtech is generally the cheapest in price but from what I've read is of good quality. From there it all seems to go up in price. I've used Hornady, Winchester, COR-BON and Buffalo Bore and it's all good but none is cheap.