It has been so long since I purchased factory ammo (other than .22 LR) that I'm not sure what the current prices are. I'd be willing to bet that .44 Mag would be cheaper than .44 Spl just because it's far more common.
Be aware that shooting .44 Spl in a .44 Mag will require a very thorough cylinder scrubbing after each session. Otherwise, a ring of powder residue will build up in the cylinder at the mouth of the .44 Spl case. If allowed to build up, it will prevent the longer .44 Mag case from chambering. My solution is to load light loads in .44 Mag brass (though I still clean everything thoroughly after each session).
The same applies to the .357 Mag / .38 Spl.