Super dooper topic. Someone should move it to a better place.
The price of a steam game depends on the game. Usually when a game is released on Steam, it is released at the same price, with quite a few exceptions. We usually see 10 dollars or more taken from Steam games, and usually alot more on sales. If you say what the game is you have in mind (if you do), that might be.. ehhh..... helpful.
And yes, all developer tools work perfectly fine with Steam games. In fact, The Steam Workshop opens up a whole new world of mods and mod tools.