Easiest way is to override the parameters in all_weapons.xml:
Code:
<?xml version="1.0" encoding="utf-8"?>
<weapons>
   <weapon file="ak47.weapon" />
   <weapon file="apc_hmg.weapon" />
   <weapon file="g36.weapon" />
   <weapon file="m24_a2.weapon" />
   <weapon file="riot_shield.weapon" />
   <weapon file="m72_law.weapon" />
   <weapon file="m240.weapon" />
   <weapon file="mossberg.weapon" />
   <weapon file="mp5sd.weapon" />
   <weapon file="pkm.weapon" />
   <weapon file="famasg1.weapon" />
   <weapon file="m16a4.weapon" />
   <weapon file="f2000.weapon">
      <commonness value="0.0012" in_stock="0" />
   </weapon>
   <weapon file="benelli_m4.weapon">
      <commonness value="0.0012" in_stock="0" />
   </weapon>
   <weapon file="pepperdust.weapon">
      <commonness value="0.0006" in_stock="0" />
   </weapon>
   <weapon file="tank_cannon.weapon" />
   <weapon file="tank_mg.weapon" />
   <weapon file="aks74u.weapon">
      <commonness value="0.0010" in_stock="0" />
   </weapon>
   <weapon file="p90.weapon">
      <commonness value="0.0007" in_stock="0" />
   </weapon>
   <weapon file="dragunov_svd.weapon" />
   <weapon file="medikit.weapon" />
   <weapon file="tow.weapon" />
   <weapon file="vss_vintorez.weapon">
      <commonness value="0.0007" in_stock="0" />
   </weapon>
</weapons>
Just set in_stock to 1 to "unrare" them; they'll be shown at armory and you'll be able to respawn with them. Similarly, you should be able to modify their price by adding "<inventory encumbrance="1.0" price="0.0" />" next to commonness tags.