Some post-apocalyptic RPG assign a high price to firearms (or their futuristic counterpart) in order to emphasize how rare such weapons have become since The End of the World. Personally, I would advertise to make the ammo for the weapons the thing that is hard to get (expensive) and give the firearms a price-tag that labels them as “affordable”.
The reason for this is “loot control”: at some point the GM will want (or: “need”) to confront the characters with (human/mutant) opponents that bear firearms, simply because the characters acquired firearms. It is very likely that the characters loot the weapons once they dealt with the opponents and that they will try to sell/barter them the next time they are in a settlement or trade post. So, if the GM uses opponents with firearms regularly, he provides the PC with a steady income. Thereby, I advise to make the ammo expensive instead of the gun itself.
The ammo is easy to keep a “cap” on. After an NPC that was equipped with a firearm was “neutralized”, it is easy for the GM to rule that there are only a few bullets left (ammo is rare) and the player characters will have to choose if they are going to keep the ammo to use it themselves or to sell it at the next opportunity. The weapon they can still sell, but it won´t make them rich. Additionally, ammo is the thing that is actually used up, so after 20 years (or more!) during which nobody produced anything (at least not on an industrial level) it makes sense that the “consumed good” is harder to get hands on than the “consuming good”.