LOL? MONEY? To make gamemodes, you need to be able to script. Learn how to script, and you will understand how to make gamemodes. And blender is completely not needed to script. It is purely a modeling program. So, generally, you shouldn't try making a gamemode unless you can script. And i might be going a bit far, but someone i know keeps annoying me by saying "Can I make addons?" "What program do i need?" "Don't you just have to open blender and type: I want an addon that when you type /gobig and then it automatically uploads it and writes description?" Sorry for sudden random writing, though it is getting annoying people that think you just do 1 little thing then you have made an addon - easy as pie. Well, hate to break it to them, but you actually have to do work, learn scripting, learn modeling for lots of addons, and so on. It is definitely not as simple as they think.
Ok then....Anyway, to start scripting, you simply make a text document, and change the file extension to .cs. I recommend starting by changing existing scripts, like the gun script, and figuring out how to change it's color and stuff, then when that is too easy, move onto something else... I found a guide which will help you quite a lot (and no, i am not just posting this to get people to look at my topic) :
http://forum.blockland.us/index.php?topic=113372.0.
I know what you must think, i was in your shoes only... a week ago, and i have learn't a LOT in those 6 days.