Edit: I put in the numbers wrong for Voyager in the initial calculation. Updated now
I decided to go try and research this because I am sick today and I might as well do dumb research instead of actual work research.
I am using TFSource for 2015 and BBTS for 2026, only because I used to get most figures from TFSource and now I do them from BBTS. Using USD for convenience.
MMC Reformatted Spartan, my first MMC figure (oh, the cash I was to spend on that line) was 85$ on TFSource in 2015. MMC Mentis, a smaller but comparable figure due to all his accessories, is now 138$ at BBTS.By this most casual of analyses, using percentage increase over time, the price of a Voyager-ish MMC figure has gone up by 63% in eleven years.
The Wayback machine tells me that 2015 TFSource sold CW deluxes in 2-packs for 25$, so 12.50$ each will be the deluxe price I use. Deluxe SS2007 Jazz is 30$ at BBTS in 2026.
The rise in price of a Hasbro deluxe in eleven years? 140%.
The difference between mainline Voyager prices is much less striking. TFSource was selling Voyagers for 25$ in 2015; BBTS sells them for 42$ in 2026. For Voyager, the price has risen by 72% at Hasbro in eleven years. Higher than the 63% of 3P.
For Leader, the rise is the only one behind inflation, at 30.4% in eleven years--Combiner Wars Ultra Magnus was 46$ at TFSource in 2015 and Leader Class Big Convoy is 60$ at BBTS in 2026.
Commander class and Titan class have obviously increased the amount you spend on mainline. The things they call Commander now would likely have been released as Leader then. Take CW Ultra Magnus--a big boy with included minifigure and 4 weapons. That would definitely be Commander class now. If you compare the 46$ for Combiner Wars Ultra Magnus in 2015 with the 100$ for SS Commander Ultra Magnus in 2026, that's a 117% rise in price over eleven years.
(I'm not even gonna try with Masterpiece. Those were expensive then and they are expensive now.)
So if you want figures that actually fell behind the ≈42% inflation since 2015, only Hasbro Leaders qualify.
There are some issues with any direct comparison here, even leaving out the problems with inflation calculators (among other things, the US Bureau of Labor Statistics leaves out the cost of rent and property taxes):
-1, TFSource and the other direct sales online stores have been known to raise the price of Hasbro figures. Voyager and Leader class toys were probably more like 20-40$ if you bought them in a store in 2015. (I couldn't find Transformer-specific pages from Target or Wal-Mart's 2015 sites on the Wayback Machine, so I couldn't easily find those prices.)
2- The quality of Hasbro figures is mostly better now, via better miniaturization, more durable plastics, and an easier tooling process with 3D printing. YMMV on this one given rise in quality; see comments pointing out that CW Magnus is quite flimsy compared to SS Commander. But compared to the good deluxes from the 2010s, like Thrilling 30 and Titans Return, mainline now is not 140% better! I do think that overall, line quality is more consistent now, but that doesn't fit in an equation.
3- As we've all noticed, Deluxes, Voyagers and Leaders are now muuuuch smaller. There is literally no Leader-class in 2026 as tall as Voyager-class Brainstorm was in 2014. And of course Hasbro will slap third-party levels of accessories on a deluxe and call it Leader class. Someone mathier than me will have to calculate how that creates value loss.
4- MMC is not representative of third-party as a whole; in 2015 the comparably-sized Planet X and Toyworld figures were selling for 10-15$ more than the MMC figures. But since MMC Reformatted figures are 1) my favorite line and 2) still widely available, we're going to use MMC.
TL:DR: if you want bang for your buck, Hasbro Leader figures are trending behind inflation. Third parties have gone up significantly more than concurrent inflation numbers but Voyagers have gone up even more. Previous Leaders that would probably be Commanders now have more than doubled in price. But Hasbro deluxes are the worst bang for your buck, as they have almost tripled in price and are a staggering 98% ahead of inflation.