I've always felt that the C-Bill cost of units were more guidelines than gospels. Good enough to get a cursory idea of how advanced and expensive hardware is, not good enough to get into all the intricacies of economies of scale, competition, supply lines, the logistics of interstellar empires, and inter-political effects on the economies of said empires. Ultimately the game's about the big stompy robots and not faceless corporate industries making the big stompy robots.
As such, a brand new prototype Giggins APC fresh from the factory probably doesn't "actually" cost the exact same 1.2 million C-Bills as a Giggins APC that's still being produced in 3145. The C-Bill system is just too rough to catch any nuances such as improved tooling and manufacturing techniques since the Giggins first entered production, since the game's not about that.
Presumably, the economies of the Houses and other political entities tends to improve over time when said entities are not being blown to bits, so the governments could also simply have significantly larger defense budgets in 3145 than they would have had in 3025, which could also explain the introduction of a lot more expensive units.
Also, the cheap-o vehicles already exist.
Scorpions, Warriors, Vedettes, Partisans and other cheap platforms have fulfilled the role of cost-effective attrition combat vehicles since the game's inception. Nothing really goes obsolete in the so Scorpions and such would still be the hardware of choice for backwater militias. Even in later eras, you can't really replace them with much, since adding anything that is
better (engines, armor, weapons) also means adding things that are
expensive, defeating the purpose of making a budget tank in the first place.
There's also a significant out-of-universe explanation for it.
Most, if not all, of the new equipment has been "high tech" goodies. Think things like the obscenely expensive XXL engine, the RISC experimental equipment, the Society's Nova CEWS, fancy-pants laser beams, what have you.
Well, say we introduced XXLs recently in MaxTech or Tac Ops or wherever it originated from. That's great, but unless you put it on a vehicle somewhere, the thing's just going to be rusting in some scientist's lab for all of eternity.
Now let's say we have a new TRO for a new era, Jihad or Dark Age or whatever's going on in the RoTS. There's only so much you can cram into the book since you're making a Technical Readout and not the entire order-of-battle of the Battletech Universe circa 3100, so you're limited to a finite number of entries. With that in mind, would you add:
- A low-tech, low-cost ICE tank whose roles are already mostly or entirely fulfilled by other existing low-tech, low-cost ICE tanks that were first introduced around the 1980s and 90s.
- A higher-tech, higher-cost tank or Mech that utilizes technology that was recently introduced in-universe, like that XXL engine, fulfilling an advanced role or niche that didn't exist before said technology was introduced.
IMO, the latter choice would be the more interesting choice 9 times out of 10.
Hence, you start seeing a lot more fancy things in the TROs like the Mad Cat Mk.IV and the Skinwalker/Ryoken III, often mounting more than one high-tech technology at a time. Part of it is it's a lot easier to differentiate the new ride from the pack if it's using a recent, rarely-used technology like XXL engines than it is to do the same with an ICE ride. Part of it is that if you don't add stuff with the fancy toys, then you have to justify devoting entire pages of a rulebook for the fancy toys when nothing canonically uses them in the first place.
That's all I have to say about that.