Well I'm not a weapons market expert.
But I do have some experience in economics.
And standard products especially on b2b markets or so do not change their prices monthly for a great variety of reasons (so calles "menu cost" is one of them).
That's neither realistic nor do I see the gain of random price increase(/decrease). Just because something CAN be done does not mean it MUST be implemented.
If you do not want the player to get drowned in money just don't let him be.
As mentioned before there are tons of possibilities to change this. More money to start with and less money per month or a higher administrative overhead or a fix inflation rate or... whatever.
The "economy" in the game is an extremely simple model (not even that) of the real world economy. In simple economic models you do not even need an inflation rate. You can just say there is a so called "present value" which takes price varieties into account.
My point in summary is:
It is not going to be realistic at any means. There are no markets (it's a list of items, no variety of trade partners, no profit margin whatsoever...). There is no real gain of randomness but a loss in the ability to plan things in detail. It is not that important to focus on standard guns because you are not going to loose because you are going to run out of assault rifles or 7.62mm NATO ammo if we want to let it look a tiny little bit realistic.
Anyway I would say the price of standard weapons should be that low in comparison to plasma guns, UFO fighters and so on that it should not even matter in late game. And therefore we do not NEED any price variation. That's just needed if we fail to adjust prices in the first place and if we do so chances are we fail to adjust price settings correctly anyway. Randomness will just add a chance of "wrong" prices int he long run or it will be that small that it does not matter anyway.
Just look at prices today.
Standard assault rifle? $3000-5000 if you want a good one? Ammo prices - lol?
In comparison standard jet fighters in huge amounts? $20-60 mio?
So if you want realism - prices for standard guns are marginal in comparison to everything else used in the game. Taking this into account I would vote for "standard gear for free"
I hope you enjoyed my overlong article once again.