You may copy, modify, and distribute the SDK and any modifications you make to the SDK in source and object code form, but only for free.
Terms of use for Valve games are found in the Steam Subscriber Agreement located here:
You may distribute your modified Valve game in source and object code form, but only for free. You may, free of charge, download and use the SDK to develop a modified Valve game running on the Half-Life engine. IF YOU DO NOT AGREE TO THE TERMS OF THIS LICENSE PLEASE DON’T DOWNLOAD OR USE THE SDK. BY DOWNLOADING AND/OR USING THE SOURCE ENGINE SDK YOU ACCEPT THIS LICENSE. PLEASE READ IT BEFORE DOWNLOADING OR USING THE HALF LIFE 1 SDK (“SDK”). THIS DOCUMENT DESCRIBES A CONTRACT BETWEEN YOU AND VALVE CORPORATION (“Valve”).
