First go in to your root\usermaps\mapname then create a new folder called gamedata and inside of that folder create a folder of weapons inside of that folder create a folder called zm.
Next locate your zm_levelcommon_weapons.csv file in root\share\raw\gamedata\weapons\zm and copy that file.
Then paste that file into root\mapname\gamedata\weapons\zm.
Then i recommend downloading a program called Apache OpenOffice and when running the installation you will have options on what to download, do ONLY Calc and nothing else only check calc.
Then edit the zm_levelcommon_weapons.csv with openoffice and make sure the menu that pops up has the lines seperated by comma, check only comma and under column "D" you will see numbers that varies between different guns, these are the prices for if there are on the wall. I've tested this myself and i changed the price for RK5 from 500 to 750.
Hope this helped!