I cannot really find the source anyways (I believe it used to be this now 404 page:
http://source.valvesoftware.com/licensing.php ).
You may want to ask about the price direcly on their website:
http://www.valvesoftware.com/contact/
Also, you can begin programming BEFORE getting a license, as long as you own a Source Engine game.
The Source SDK 2013 is on Github:
https://github.com/ValveSoftware/source-sdk-2013
From Steam's Tools page you can also download the prebuilt stock HL2 binaries for your system (Source SDK Base 2013 Multiplayer, Source SDK Base 2013 Singleplayer and Source SDK Base 2013 Dedicated Server).
Just remember, your "game" will be considered as a MOD.
If you get a commercial license you should be able to sell it as a standalone game anyways (See: Garry's Mod).
A basical list of things to do is this:
1. Download the Source SDK 2013
https://developer.valvesoftware.com/wiki/Source_SDK_2013
2. Download the Source SDK Base 2013 MP/SP
https://dl.dropboxusercontent.com/u/83943521/Screens/f/cplpl/srcsdkbase.png
3. Create a Mod (Gamemode):
https://dl.dropboxusercontent.com/u/83943521/Screens/f/cplpl/sdkroute.png
Duplicate the hl2mp folder, renaming it in your short-name for the mod
Enter the folder and open gameinfo.txt
Replace "hl2mp/" with "yourmodname/"
Change the name and title to your liking
Save and close
Go in the Source SDK Base folder, enter the bin directory
Open hammer.exe:
Hammer.exe is the official map editor for the Source Engine.
You will be asked a configuration: Choose hl2mp.
Open Tools->Options.
Near Configuration, choose Edit.
Make a new Gamemode configuration for your game.
Close the Edit Game Configurations window.
Under Game Data files:
Remove all the elements from the list, and add base.fgd and hl2mp.fgd
Based on how many files are in the list (and their content) you will have different entities to use in the Hammer Editor, but you must make sure the entities are in your game project, or errors will occur.
Make sure Texture Format and Map Type are for Half-Life 2.
Choose your Default values.
I suggest a func_door_rotating for SolidEntity, 0.25 for Texture Scale and 16 or 8 for Lightmap scale.
PointEntity should be the respawn point (mine is info_player_start, but this should be for the singleplayer mode).
Point the Game Executable Directory to your Source SDK Base directory and Game Directory to the folder you made earlier.
The Hammer VMF Directory will be the default folder where your maps will be saved.
Now, choose the Build Program tab.
Choose your Configuration.
in Game Executable, choose the hl2.exe in the SourceSDK Base directory.
For the BSP Executable, find vbsp.exe in the bin directory.
VIS = vvis.exe
RAD = vrad.exe
Place compiled maps [...]:
Source SDK Base\your gamemode\maps
|
4. Make a test map
https://developer.valvesoftware.com/wiki/View_Navigation
5. Get your hands dirty
https://developer.valvesoftware.com/wiki/My_First_Mod
6. Test
To run your mod, place the compiled files (client.dll and server.dll) in:
SourceSDK Base\yourgamemode\bin
Then open the console at the SourceSDK Base directory, and run:
hl2.exe -game yourgamemode
(You are free to make a .bat file for that).
This is just the bare bones, I can't tell much more.
If you need more, you can search here:
https://developer.valvesoftware.com/wiki/SDK_Docs