Double checked on SSW, the Mad Cat IV Prime is almost 92 Million C-Bills. Not even the most Expensive Mech either.

You could buy an entire company of mechs with that money.
Assuming you get a small discount for buying in bulk, you could field an entire
regiment of Urbanmechs with that kind of money.
Get yourself a Wyvern IIC and tinker with it a bit. It already has Arena 'Mech aesthetics,
and the crotch-mounted gun.
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Bonus points for pissing off the Clanners by turning one of their 'mechs into a dedicated melee platform.
If it's dick-gun you want, how about the Marshal?

The dick-gun flamer might be short and stubby, but when you set fire to an entire platoon of infantry while doing hip-thrusts, that does make up for the size I say.
I understand why it happens, since it would be impossible to run unit prices otherwise, but equipment prices being exactly the same all the way from 2750 to 3150+ always makes me chuckle. No matter how many factories are building them, no matter how commonplace the design is, no matter the improvements in production technology... those medium lasers are going to cost 40,000 C-bills a pop and that's that. 400 years, Four Succession wars, a Clan invasion, two huge Civil Wars and a Jihad, not a drop of inflation. Now that's a powerful economy.
Really, BattleMechs have no depreciation over time and the parts always cost the same, so it wouldn't be a bad investment to just buy a bunch of 'Mechs, mothball them somehwere safe, and then flip 'em for a higher price whenever the next "it all went to shit and we need 'mechs right the fuck now!" event rolls in. Actually, that sounds suspiciously like the Diamond Sharks...
There are traits for mechs that kinda take this into account:
Obsolete and ubiquitous make it harder/easier to find spare parts, which might also translate into making them more expensive or cheaper.
Some stuff, like MLasers, is so commonplace, I wouldn't expect price fluctuations, since even when the production facilities are destroyed in some area, there's always a bunch of mothballed pieces lying around somewhere or being hauled around by traders.
But I think, if one would want to houserule it, it wouldn't be too hard to come up with a convenient and quick to use system.
Essentially, you break the timeline down into eras (Age of War, Star League, Early Succession Wars, etc.) and increase the price by a certain amount for stuff in the era that it was introduced. As long as it stays in production to a reasonable extend, the price is the usual list-price (you might drop the price for particularly common stuff), when something goes out of production, you increase the price for every era that it wasn't made by a certain amount, until it enters production again.
You could just as easily add a basic system to reflect that certain things are only made by certain factions.
You assume the listed price for the state that it was made in and add a certain percentage for every border it has to cross (or based on the distance to the state). If it's something that is usually restricted to, say, royal units, increase the price some more.
It would be fiddly to find the right amount of price increase I guess, but overall, you don't have to make an ordeal out of it.
Bit of a sidenote, but in the HBS game, I sometimes clean up my storage by selling mechs and equippement that I don't need and it always makes me smirk, when I imagine how I ruin the prices for mechs and weapons for years to come in that system by dumping an entire battalion's worth of mechs and hundreds of tons of weapons and equippement.