7mm-08 ammo has to be cheaper than 7 mag, by a bunch.
I can tell you that you don't want to get a 7-08 as it will do everything that you 7mag will do, except kick and dot your-I (cut you). The bullets carry well (high Ballistic Coefficent) and it is very accurate. The whole family is very accurate. Once you see how well that "little" 7mm shoots, you'll chuck that 7mag.
BTW, as a side note...Jack O'Connor's wife hunted side by side w/him, the world over w/a 7x57 and he thought that it was a pretty darn good gun. She knocked all sorts of critters doa. Also, as a side note, his loads for her were way off the chart. Those loads were a twin to the 7-08... or a bit more. She shot Mtn goats, antelope and probably African game w/it.
The .243 is plenty up to any deer sized critter and long range shooting. Accurate, high fps, and easy to shoot.
Now, the one that is possibly better than both...right smack dap in the middle...260. The bullets carry exceedingly well nad the 120 grainer is used in Europe for all manners of beasties for crtters up to 500# as the 6.5. But, not every manufacturer builds ammo for it.
You haven't mentioned the range...Eastern deer, or Western deer. Out to 100 yards...who cares. Out to 300, who cares. Farther than that, it matters...a bit. If you hit em square, they still fall over dead.
You have the .243 and really need another gun...go get the 7-08.