If you don't already cast, the cost to get you started far outstrips the expense of buying jacketed bullets for reloading, unless you plan on shooting thousands of rounds out of your 45/70.
I cast 405gr FP, and also load with Hornady 350gr HP bullets.
I think that a lot of the cost depends upon the equipment you choose to use!
You can go high tech and spend a bunch. Or, can cast similar to the way that our forefathers did. Casting good accurate, quality bullets won't bankrupt you…..and you may find it a very pleasurable addition to your shooting game.
A simple, small cast iron ……probably can be bought used for near nothing at a second hand store or pawn shop. A casting dipper > under $40.00.
And then the bullet molds……. Lee offers many different aluminum molds at pretty darn reasonable prices. I presently use molds from Accurate Molds. He offers iron, brass, or aluminum. He will custom size the mold according to the casting alloy that you use. "AND"……you can get a mold of two different calibers or two different weights or designs in the same caliber. Considering the quality and options that he offers….they are very reasonably priced. Several years ago I bought a 4 cavity aluminum mold from Accurate, 2 of the cavities are for my 45-70 and 2 of the cavities are for my 460 S&W.
For many years I did my casting in our kitchen. I simply covers the entire stove top other than the burner/element that I was using to keep any and all molten lead splatters off of the stove top. And…..most everyone's stove already has a vent fan to draw away unwanted fumes.
Sizing …..often no sizing is required. If you need or desire, you don't need the sizer/lubricator specifically made for that….. for under $20, you can buy a sizing die from Lee that simply screws into your present reloading press.
There are several methods to lube your bullets without additional or expensive equipment. Or, you can get into powder coating…..no messy lubes to deal with, and you color code your bullets to specific calibers/weights, ect. Powder coating virtually eliminates the fouling/leading usually associated with cast bullets……making them comparable to shooting jacketed bullets. Also, powder coating bullets is not an expensive endeavor. The power is inexpensive, there are recommended brands - don't buy from Harbor Frieght, and many people use a simple electric toaster over with can also be bought quite cheap in 2nd hand stores or pawn shops.
There is tons of information both in books and via the internet providing you with more information than you can absorb. memtb