Focusing on the OP's original question:
"Here's my question: For the average guy on the street who lives paycheck to paycheck and who wants a handgun for self-protection and plinking, has the .40 S&W (and maybe the .45 ACP) become more relevant due to ammo prices? The price of .38/.357 is almost cost prohibitive, reaching "big bore" prices. So that would push me away from a revolver if I were so inclined and towards an auto. If I bought into the premise that you need something with a little more umphh than a 9mm, wouldn't the .40 S&W be a more logical choice based on availability, power, and price? From that perspective, it would seem to me that the .40 S&W becomes more relevant than ever before. Your thoughts?"
For the first time gun buyer (which very few of us are) I would recommend a polymer 380 or 9mm. As much as I enjoy my revolvers, a quality S&W would cost 2-3 polymer 9s, and the OP cites an owner living paycheck to paycheck. More so because of ammunition supply versus ammunition demand, 9mm has become the 38 Special wadcutter load. I have no argument with the 40 (I have 2), it's just that ammo costs could prevent the purchase of life essentials in order to practice.
Under normal situations (if such an animal still exists) I'd recommend the purchase of the selected pistol, a few boxes of SD ammo, several boxes of range ammo, then a reloader to make your practice ammo and replicate the SD ammo. However, that Dillon Square Deal B that I bought in '89 for $198 is now selling for between $700-1000!
I would still recommend that the first time owner get a polymer 380 or 9mm. My preferred target 9s are Sigs and S&W pistols, but I also shoot Glocks. Get the pistol, save the brass, and eventually reload. My advice for the paycheck to paycheck first time gun owner is to join the GSSF ($35), get the coupon, and look at either a: G42 (380 for $340), a G43 (9mm for $380) or any of the Gen5 9mms for $425 (non-MOS) and start your adventure. I know that entry Sig p365s can be had for about $500, as can some Shield Plus pistols. Either would be a sound start.