I think - though I'm not positive - that the game server knows how much items cost for the purposes of buying and selling, but the client is what displays the menu showing how much things are. The information isn't passed through from the server to the client.
If that's the case, then your modification to change the cost of weapons will work server-side, but clients will still see the original cost of them. Unless you build a new cgame/ui and package them as a pk3 for clients to download, but most people will not bother downloading a pk3 for something like that (ie, your server will not see much traffic unless people *really* want to be there).
Check the source to verify if I'm just talking out of my rear before you believe it, I'm too lazy to do so right now
