the names of cartridges can get confusing, especially if yah start chatting about the old black powder rounds as the Brits named stuff differently than the US. ok with the fact that the 45-70 is a 125-127 years old
45=45 caliber bullet, .458-ish inch actually, i say -ish because alot of the old guns had bores that varry a bit from each other and it generally actually falls in using a .457-.459 inch bullet, with my modern rifle i run .458 jacketed bullets and .459 cast bullets through my gun.
70=70 grains of black powder which was the original load, i use smokeless so it means nothing to me other than its the name of the cartridge.
really want to confuse thing? the 45-70-350 and 45-70-405 are all names for the same 45-70 round just with different bullets. things was confusing 100 years ago. even more screwy the 50 Express, 50-100, 50-105 and 50-110 are actually all the same round with different loads of black powder in the same case.
the Brits did things differently, the 577/450 used the old 577 case with a 45 caliber bullet. case comes before bullet
then yah got the 45 colt which uses everythign from .451 to .454 bullets but not the .458 bullets of the 45-70, 458 Winchester Magnum, 458 Lott, ect. the 44 mag actually uses bullets that are .429 inches wide. the 38 special and 357 magnum both use bullets that are .357 inches as do the 35 cal rifles such as the 35 Whelen and 350 Rigby. the 7x57 Mauser and 275 Rigby are the same round.
lots of variation. really want to blow your mind on the number of rounds that are/was out there? pick up a new printing of Cartridges of the World by the late Frank Barnes, its a $20-$25 book that is entertaining to flip through and he still missed some.
as for cost. varries greatly. i havent started relaoding for my 223, i prefer to pay $10-$20/50 rounds then mess with those dinky things. i reload my 45-70 shells for around $.50 each sometimes less if i have used the same case several times but i buy new brass frequently, a box of new rounds from the store would be between $20 and $50/20 depending on brand and load. i run bullets through my various guns that cost anywhere from $.10 to $1.50 each thats before brass, powder and primers. reloading dies run between $19 a set for something like basic 30-06 dies to the $260 a set ill spend on for the 50-110 if i ever get it built though between $20 and $50 is average. my lil hand held reloading press cost $25, hand held primer seater cost $15, and my best investment so far the electric powder dispenser/scale cost $300(sold a rifle to buy it and am VERY glad i did).
if you want to get a good idea of whats out there for a given round, check out Midway USA, they have most of the stuff your going to need and if they dont have it chances are its going to take quite a bit of searching to find.