at the risk of oversimplifying an answer for you.
Here's two loads, selected for easy comparison:
- 40S&W: WSP, 5.0 gr. 231, 180 LTC
- 45ACP: WLP, 5.0 gr. 231, 180 LSWC
FWIW, these are max loads from the Hodgdon site.
primer: cost is the same.
powder: cost is the same.
bullet: arguably, since weight is the same, cost is the same.
case: This is where you would find the variable. This load runs about 16,500 PSI (?) in the 45ACP, and 32,900 in the 40S&W. In short, the brass for the .40S&W won't last as long as the 45ACP brass.
From what I read on the forums, people can find plenty of reloadable 40S&W brass. I'm guessing 45ACP is found less often. So, arguably, 45ACP will cost you 'more.' However, the 40S&W brass will wear out sooner. So, maybe that's a wash, too. I can tell you I would be looking hard at 40S&W brass after five reloadings for bulges and splitting. The 45ACP brass would last "indefinitely"--twenty times or more.
Just in terms of reloading 'ergonomics', I find the 45ACP components much easier to handle, and much less finicky. Another .005-deeper seating on the .40S&W might shorten the brass life noticably (if not cause other problems), while the .45ACP wouldn't change, really--except to become more (or less) accurate.
If you are new to reloading, I think you would find the 45ACP to be a much better cartridge to learn on.
Jim H.