I can get you close. In 1902, Sears & Roebuck would be selling 45-70 with 405 gr bullets for $2.54 per 100, or .54 per 20.

12ga shot shells, 3" , black powder, loaded with 1 oz of shot would be .$1.29 per hundred, .34 per 25.

Interestingly, you could also buy the empty 12ga. paper shells from Sears, unprimed, for .80 per hundred, and load them how you liked, and even get all-brass 12 ga. shells for .08 each to do the same. In the 1890's, it would have been possible to buy the new smokeless shells, but not buy your own smokeless powder to reload them.

EDIT In 1902 the Dick Act would have just allowed huge numbers of 45-70 Trapdoor rifles to be dumped out of the militia units, to be replaced by regular army rifles. For many decades they would be seen for sale for about $2 at hundreds, if not thousands, of small stores across the US, so it's possible that might have affected the price of ammunition, compared to, say 1895. But in 1927 Bannerman's would be selling 45-70 at $3.00 per hundred, out of crates of 1,000.